Drug metabolism is generally split into two phases based on what actually happens to the molecule. Phase-I reactions modify the drug's existing structure, usually by adding, removing, or exposing a functional group such as -OH, -NH2, or -COOH. Phase-II reactions go a step further and attach the drug, or its Phase-I metabolite, to a small water-soluble endogenous molecule so the body can excrete it more easily.
Oxidation, reduction, and hydrolysis all fall on the Phase-I side. Oxidation, driven mostly by cytochrome P450 enzymes, is the workhorse Phase-I reaction. Reduction and hydrolysis do the same kind of job through different chemistry, breaking or converting a bond within the drug molecule itself, without attaching anything new to it.
Acetylation is different in kind, not just in mechanism. An acetyl group from acetyl coenzyme A is transferred onto the drug by N-acetyltransferase, which is a true conjugation step, joining the drug to an outside chemical group rather than simply altering what was already there.
Because conjugation reactions define Phase-II metabolism, acetylation is the answer that fits, and none of oxidation, reduction, or hydrolysis do.