Hint: Chlorophyll is a photosynthetic pigment and has a tadpole-like structure. It comprises a phytol tail and a porphyrin head. It harnesses the light energy to synthesize ATP which is used for numerous cellular processes.
Complete answer: Porphyrin head: Chlorophylls have a complex ring structure chemically related to porphyrin-like groups found in hemoglobin and cytochromes. It is usually the site of electron rearrangement when the chlorophyll is excited. It is a cyclic tetrapyrrole structure having four heterocyclic pyrrole rings with non-ionic magnesium atoms. Phytol tail: It comprises a long hydrocarbon tail almost always attached to the ring structure and it anchors chlorophyll to the hydrophobic portion of the thylakoids. Nitrogen is a mineral element required by plants in the greatest amount. It is particularly required by meristematic tissues and metabolically active cells. It is one of the major constituents of proteins, nucleic acid, vitamins, and hormones. It is also a constituent of the pigment chlorophyll. Hydrogen and carbon are the non-essential mineral elements that are indispensable for plant growth. Carbon and hydrogen, along with oxygen constitute about 94% of the total dry weight of the plant. They take part in the synthesis of the cell walls, storage carbohydrates, protoplasmic constituents, and other biomolecules. Hydrogen not only takes part in oxidation-reduction reactions but is also an important cation as well as a determinant of pH. Iron is a micronutrient and is required in larger amounts in comparison to other micronutrients. It is an important constituent of proteins involved in the transfer of electrons, activates catalase, and some other enzymes. It is very much essential for the formation of chlorophyll and other pigments.
Therefore, the answer is (D) Iron.
Note: - Iron is absorbed as ferric ions. - Magnesium is present in the ring structure of chlorophyll. - Chlorophyll containing chloroplast is a type of plastid.