In the citric acid cycle, the reaction which produces succinyl CoA also forms carbon dioxide. The key intermediate enzymic step involves:
The enzyme is a-ketoglutarate dehydrogenase, an a-keto acid, which is decarboxylated. Therefore the second choice is correct. The only confusion might be with the last choice, since in effect it involves the oxidation of a keto group to a carboxylic acid. However, the product is the thio-ester of a carboxylic acid.
The decarboxylation of a hydroxy acid
The decarboxylation of an alpha-ketoacid
The decarboxylation of a beta-ketoacid
The oxidation of a keto group to a carboxylic acid