Final answer:
Diacetylmorphine is acylated with acetic acid to form heroin. Acetic acid's prefix 'acet-' indicates it corresponds to a compound with two carbon atoms.
Step-by-step explanation:
The question asks which acid is used to acylate diacetylmorphine. The process of acylation involves introducing an acyl group into a molecule. Diacetylmorphine, commonly known as heroin, is synthesized from morphine through acylation with acetic acid. As we look at the prefixes given in the question, 'acet-' corresponds to a compound with two carbon atoms, indicating acetic acid. Morphine is converted into diacetylmorphine, also known as heroin, when both of its hydroxyl groups are turned into esters of acetic acid.