Biological Importance of Elements - Na, K, Mg, and Ca

$Na^{+}$,$K^{+}$,$Mg^{+2}$ and $Ca^{+2}$ ions are concentrated in animals.

Hypokalemia is when blood's potassium levels are too low. Potassium is an important electrolyte for nerve and muscle cell functioning, especially for muscle cells in the heart. Your kidneys control your body's potassium levels, allowing for excess potassium to leave the body through urine or sweat.

Plants need a small amount but high concentration of K for specific functions in the cytoplasmand a major portion (∼90%) of it is localized in vacuoles, where it acts as an osmoticum. Maintenance can be achieved by other cation like sodium too .

The daily requirement of sodium is $600 \ mg$.

As sodium accumulates, the body holds onto water to dilute the sodium. This increases both the amount of fluid surrounding cells and the volume of blood in the bloodstream. Increased blood volume means more work for the heart and more pressure on blood vessels. Over time, the extra work and pressure can stiffen blood vessels, leading to high blood pressure, heart attack, and stroke. It can also lead to heart failure.

In C4 plants, sodium is a micronutrient that aids in metabolism, specifically in regeneration of phosphoenolpyruvate (involved in the biosynthesis of various aromatic compounds, and in carbon fixation) and synthesis of chlorophyll.

The minimum requirement of potassium per day for an adult is $4700\ mg$.

Option $(A),\ (B)$ are correct.

Option $(A),\ (B)$ are correct.

Option $(B)$ is correct.

${ Ca }^{ 2+ }$ is the ion helpful for controlling heart beat and muscle contraction.

Calcium ions (Ca2+) are crucial for muscle contraction, including the rhythmic contraction of cardiac muscle (heartbeat). Therefore, the statement that they are not important in maintaining the regular beating of the heart is false.

Simple proteins bonded with a non-proteinic prosthetic group (acting as cofactor) are called conjugated proteins.
Examples include  Glycoprotein, Lipoprotein, and Phosphoprotein.

Calcium carbide (CaC2) reacts with moisture to produce acetylene gas, which acts as a synthetic plant hormone to induce fruit ripening.

For fixing atmospheric nitrogen -nitrolim is used and further $CaCN _2+C$ is used as fertilizer.

$Mg$ is present in chlorophyll which is a green pigment and is used in photosynthesis in green plants.

Hence option B is correct.

Calcium carbide is also used for ripening fruit artificially in some countries. When calcium carbide comes in contact with moisture, it produces acetylene gas, which is quite similar in reaction to the natural ripening agent ethylene.

Magnesium is the central metal ion in the porphyrin ring of the chlorophyll molecule, which is essential for photosynthesis.

Holoenzyme = Apoenzyme+ Cofactor.

Total hardness is calculated based on the concentration of Ca2+ and Mg2+ salts expressed as CaCO3 equivalents. Using molar masses (Mg(HCO3)2=146, Ca(HCO3)2=162, CaSO4=136, MgCl2=95, CaCl2=111), the calculation yields 300 ppm.

$Ca^{2+}$ ions are required for contraction of muscles in humans.

Mg binds to phosphate group in ATP thus making a complex that catalyses phosphate transfer. 

The chief component of the nacre that constitutes the pearl is aragonite $(CaCO _3)$. Nacre also contains a small amount of conchiolin, a hornlike organic substance (albuminoid) that is the main constituent of the mollusk's outer shell.

Mg2+ is indeed a cofactor for phosphate transfer enzymes (like kinases) and is the central atom in chlorophyll, which is used in photosynthesis.