The correct option is B magnesium atom
Chlorophyll is a pigment that is found in the chloroplast of photosynthesizing plants. These pigments are porphyrin molecules. Porphyrin is an organic molecule. In humans, haem is a porphyrin molecule. Like how the haem is associated with a metal ion (Fe) similarly the chlorophyll in plants is associated with a metal ion which is magnesium. Porphyrin head is a complex ring that acts as the site of electron rearrangement with a magnesium atom at the center.