Which action could produce a carbonyl group?
a. the replacement of the —oh of a carboxyl group with hydrogen
b. the addition of a thiol to a hydroxyl
c. the addition of a hydroxyl to a phosphate
d. the replacement of the nitrogen of an amine with oxygen

a. the replacement of the —OH of a carboxyl group with hydrogen

Explanation:

By reduction methods a compound with a carboxylic acid group can be reduced. By adding H2 to said carboxylic group, some catalyst will be needed to allow this reaction to progress (in some cases metal palladium is usually used as a catalyst)

Before replacing the OH group, it is usually reacted to form an acyl chloride, which, when it is reduced with H2 and the catalyst is transformed into an acyl chloride and then rapidly reduced to form the carbonyl group and therefore the aldehyde

i think the answer is b.