a. replacement of the -OH of a carboxyl group w/hydrogen
b. addition of thiol to a hydroxyl
c. addition of a hydroxyl to a phosphate
d. replacement of the nitrogen of an amine with oxygen
e. addition of a sulfhydryl to a carboxyl
Unfortunately, this is a poorly worded question. Answer a is probably the best answer of the five given. However, replacing the OH of a COOH with an H to make an aldehyde does not actually create a carbonyl group. The carbonyl group, which is a C=O, is already present in the COOH. One could argue that the substitution of OH by H makes a carbonyl compound, an aldehyde, as opposed to a carbonyl group, but the difference is semantic because some people consider only aldehydes and ketones to be carbonyl compounds, while others consider compounds containing any functional group that includes the C=O, such as carboxylic acids, amides, esters, etc., to be carbonyl compounds.
Only the first: R-CO2H —> RCOH