Minerals and Nutrients

Correct answer is (4).

Chlorophyll is a porphyrin-based molecule with magnesium (Mg) at its center, analogous to heme's iron core. The magnesium ion is essential for light absorption and electron transfer during photosynthesis. Calcium, copper, and iron are not the central metal in chlorophyll.

Water is most abundant and important constituent of protoplasm. All the metabolic reactions of a body take place in water medium.

Sulfur is an essential mineral contained in amino acids cysteine and methionine, which are necessary for protein synthesis. While sodium, iron, and potassium are important minerals, they are not specifically required for protein structure formation. Proteins contain sulfur but not the other listed elements as structural components.

Magnesium is the correct answer. It forms the centre of chlorophyll in plants.

Insulin contain Zn, haemoglobin contain Fe, vitamin B₁₂ contain Co and chlorophyll contain Mg.

Zn is a micronutrient. Micronutrients such as boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn) are essential for plant growth. If the micronutrient supply is insufficient, plants may develop deficiency symptoms and yields may be reduced. 

Carbohydrate is a by-product of food prepared by the plants. It is not required by the plants to produce food.

Chlorophyll is a green coloured pigment that is present in green leaves and absent in yellow leaves. It helps to make food in the auto-trophic plants.

Plants make their food in the form of carbohydrates.

It is a metal in the first transition series.

Fertilizers primarily supply three essential macronutrients to plants: Nitrogen (N), Phosphorus (P), and Potassium (K) - collectively known as NPK. Sodium (Na) is not a primary plant nutrient and is not typically added through fertilizers. In fact, excess sodium can be harmful to most plants.

Iodine is an essential component of thyroid hormones T3 (triiodothyronine) and T4 (thyroxine), which regulate metabolism, growth, and development. Iodine deficiency causes goiter (thyroid enlargement). Phosphorus is important for bones and ATP, sodium for nerve impulses, and calcium for bones and muscle function - but none are specifically required for thyroid hormone synthesis.

Iron is the central component of haemoglobin, the pigment that gives red blood cells their red colour and enables oxygen transport. Each haemoglobin molecule contains four iron atoms that bind to oxygen molecules. Phosphorus, potassium and magnesium are essential minerals but they do not form part of haemoglobin structure.