(D) is incorrect. Hydrogenation of vegetable oils is an addition reaction that uses a nickel or palladium catalyst, not an acid catalyst.
The passage clearly states that unsaturated hydrocarbons add hydrogen in the presence of palladium or nickel catalysts — no acid is involved. Options A, B, and C are all correct statements about hydrogenation. Examiners expect you to identify the specific wrong detail (acid catalyst) in option D.