Chlorophyll b can be converted into chlorophyll a by replacing:

Explanation

Option A is correct. Chlorophyll b is converted into chlorophyll a by replacing the carbonyl group (formyl group) with a methyl group on the porphyrin ring. This enzymatic process, facilitated by protochlorophyllide oxidoreductase, is crucial in photosynthesis, allowing plants to utilize a wider spectrum of light for energy conversion. Options B and C are incorrect because they suggest replacing the central magnesium atom, which is integral to chlorophyll's structure, with ferrous, which is related to hemoglobin and not involved in chlorophyll structure. Option D is incorrect because a specific conversion process exists between chlorophyll b and chlorophyll a.