Diversity in Living Organisms

The average diameter of bacteria is 0.2 to 2 micrometer. 
Mitochondrion size is 0.5 to 10 micrometer. 
The average diameter of a mammalian cell is 20 micrometer. 
Virus diameter ranges from 20 nanometers to 250-400 nanometer. 1000 nanometer = 1 micrometer. Hence, the virus particle is the smallest of all.

In unicellular, reproduction occurs asexually and two new identical daughter cells that are produced. All multicellular start their life from a single cell i.e., zygote which is a product of the fusion (fertilization) of usually two haploid gametes, a female and a male gamete. The zygote (diploid) divide to give rise huge variety of different types of cell, specialized for different functions which in turn forms multicellular organism.

A coenocytic organism is a multinucleated cell that can result from multiple nuclear divisions without cytokinesis. Rhizopus is a classic example of a coenocytic fungus.

As organisms grow larger, they generally increase in cell number rather than individual cell size, as larger cells face surface area to volume ratio limitations.

An organism which performs and exhibits all functions of life via a single cell, it is an unicellular organism. Such as prokaryotes like bacteria, eukaryotes like yeast are unicellular organisms. They lack conglomerate of cells (tissue) or organ systems to perform specific functions. All the cellular functions for it to be living are performed by the individual cell.  So, the correct answer is 'Unicellular'.

Reproduction is the characteristic for living organisms i.e., both unicellular and multi cellular organisms can reproduce irrespective of their cellular level. But, only multi cellular organisms have specialized organs for digestion and respiration/breathing that is GIT (Gastro Intestinal Tract) and Respiratory tract respectively, unlike unicellular organisms who respire and engulf food by general body surface. Thus, digestive tract and respiratory tract are defining feature of multicellular organism (ruminant). 
So, the correct answer is 'Both A and C'.

Some living organisms are made up of once cell only, these are called unicellular. These organisms have a large surface area to volume ratio and rely on simple diffusion to meet their needs. An example of a unicellular animal is Amoeba. Amoeba feed on smaller organisms such as bacteria.

Amoeba is unicellular because it consists of a single cell (B) and lacks specialized organs (C), performing all life processes within that single cell.

The organisms can be broadly classified into two groups, namely unicellular organisms and multicellular organisms depending upon the number of cells. Unicellular organisms are made up of only one single cell. Examples include Amoeba, Paramecium, yeast, bacteria. Multicellular organisms are those organisms that are made up of more than one cell. Examples include human beings, animals.

Receptors that sense the change in atmospheric pressure are classified under mechanoreceptors. Three pressure receptors are namely Pacinian corpuscles, Ruffini endings and Krause end bulbs which are located in skin. This makes option A incorrect. The three semicircular canals lie at right angle to each other in vestibular apparatus of inner ear and are filled with endolymph; this arrangement senses the change in movement (acceleration or deceleration) or position. The mechanism includes movement of cupula in response to movement of endolymph within the canal causing the bending of cilia to generate the nerve impulse. This makes option C correct and option D incorrect. Migratory birds and animals have receptors in their brain which are activated by direction of sunlight and orient their body at certain angle in relation to sun. This makes option B incorrect. Correct answer is C. 

Receptors that sense the change in atmospheric pressure are classified under mechanoreceptors. Three pressure receptors are namely Pacinian corpuscles, Ruffini endings and Krause end bulbs which are located in skin. This makes option A incorrect. The three semicircular canals lie at right angle to each other in vestibular apparatus of inner ear and are filled with endolymph; this arrangement senses the change in movement (acceleration or deceleration) or position. The mechanism includes movement of cupula in response to movement of endolymph within the canal causing the bending of cilia to generate the nerve impulse. Migratory birds and animals have receptors in their brain which are activated by direction of sunlight and orient their body at certain angle in relation to sun. Thus, the correct answer is option C.

Polar bears inhabit the Arctic (North Pole), while penguins are native to the Southern Hemisphere (mostly Antarctica). They do not coexist in the wild.

Aestivation is a state of animal dormancy characterized by inactivity and a lowered metabolic rate in response to high temperatures and arid conditions.

The five kingdom classification was proposed by R.H. Whittaker in 1969. The five kingdoms were formed on the basis of characteristics such as cell structure, mode of nutrition, the source of nutrition and body organization. It includes Kingdom Monera,  Protista, Fungi, Plantae, and Animalia. According to Whittaker's 5 kingdom classification, unicellular eukaryotes belong to kingdom Protista. Amoeba and euglena are unicellular eukaryotic organisms so they belong to kingdom Protista.