Choose the incorrect statement about the common reaction used in hydrogenation of vegetable oils.

The NCERT chapter states explicitly: 'Unsaturated hydrocarbons add hydrogen in the presence of catalysts such as palladium or nickel to give saturated hydrocarbons... This reaction is commonly used in the hydrogenation of vegetable oils using a nickel catalyst.' The question asks for the INCORRECT statement, which is option D.

Why D is incorrect (i.e., the correct choice for this question): Hydrogenation of vegetable oils is carried out in the presence of a nickel (or palladium) catalyst, not an acid catalyst. Acid catalysts are used elsewhere in the chapter for a completely different reaction — esterification, where ethanoic acid reacts with ethanol in the presence of a small amount of concentrated sulphuric acid (an acid catalyst) to form an ester. Mixing up these two catalysed reactions is the trap in this question; hydrogenation specifically requires a metal catalyst such as nickel or palladium, so calling it an 'acid catalyst' reaction is factually wrong.

Why the other options are correct statements (and therefore not the answer):

• Option A: Hydrogenation is indeed an addition reaction — hydrogen atoms are added across the carbon–carbon multiple ($\mathrm{C=C}$) bonds present in the unsaturated fatty acid chains of vegetable oil, converting them to single ($\mathrm{C-C}$) bonds. This matches the chapter's classification of hydrogenation as an addition reaction of unsaturated hydrocarbons.
• Option B: The chapter directly names nickel and palladium as the catalysts used for this hydrogenation reaction, so this statement is correct.
• Option C: Since hydrogenation converts the $\mathrm{C=C}$ double bonds of the unsaturated oil into $\mathrm{C-C}$ single bonds by adding hydrogen at every double bond, the resulting product (a saturated fat, e.g. vanaspati ghee) contains only single bonds between carbon atoms, consistent with the text's statement that 'vegetable oils generally have long unsaturated carbon chains while animal fats have saturated carbon chains' — hydrogenation converts the former into the latter type of chain.

Hence, the only false statement among the four is D, making it the correct answer to this 'choose the incorrect statement' question.