The correct option is
A Chlorophyll
Magnesium is an essential plant macronutrient. It is a component of green pigment chlorophyll.
Chlorophylls have a porphyrin head and a long phytol tail. The porphyrin head is a cyclic tetrapyrrole structure in which four pyrrole rings are coordinated to central magnesium atom.
Deficiency of magnesium leads to interveinal chlorosis, i.e., yellowing of leaves and other plant parts.