**Correct option (a)Aldehyde group**
Explanation :
Cholophyll has a tetrapyrrole porphyrin head and a long chain alcohol called phytol tail. Each pyrrol is a 5 member ring with one nitrogen and four carbon. A non-ionic Mg atom **** in the centre of porphyrin, attached to nitrogen atoms of pyrrole rings. Chlorophyll -α has methyl group at carbon 3 of pyrrole ring and Chlorophyll -β has formyl (aldehyde) group attached to this atom.