Which of the following reactions does the coenzyme biotin catalyze? a. carbon adjacent to an ester or keto group.
b. decarboxylation of an a-carboxy acid.
c. decarboxylation of an a-keto acid.
d. carboxylation of an a-carboxy acid.
e. decarboxylation of a carbon adjacent to an ester or keto group.

c. decarboxylation of an a-keto acid.

Explanation:

Decarboxylation refers to the removal of the carboxyl group from a carboxylic acid and thus releasing carbon dioxide. Decarboxylases are enzymes that speed up the removal of the carboxyl group from acids. These reactants could be amino acids, alpha-keto acids, and beta-keto acids. Biotin is known to catalyze the decarboxylation of malonyl CoA to acetyl CoA during fatty acid synthesis.

Malonyl CoA is converted to acetyl CoA after decarboxylation assisted by biotin also known as Vitamin H. Alpha keto acids are involved in fatty acids synthesis and Malonyl CoA is an alpha-keto acid because the keto group is located in the first carbon near the carboxylic acid group. Keto acids have both a carboxyl group and a ketone group.

it's all four options.