The major product formed in the reaction of benzoic acid with isobutylene in the presence of catalytic amount of sulphuric acid is A.
The reaction is an example of an Friedel-Crafts acylation. In a Friedel-Crafts acylation, an alkyl halide or an alkene is reacted with an acyl halide, an anhydride, or a carboxylic acid in the presence of a Lewis acid catalyst to form an alkyl or aryl ketone.
In this case, the Lewis acid catalyst is sulfuric acid. The sulfuric acid protonates the oxygen of the carboxylic acid, making it more electrophilic. The isobutylene then attacks the protonated carboxylic acid, forming a tetrahedral intermediate. The tetrahedral intermediate collapses, expelling water and forming the acylium ion. The acylium ion then attacks the benzene ring, forming the ketone product.
The other options are incorrect because they do not represent the major product of the reaction. Option B is the product of a Friedel-Crafts alkylation, which would not occur under the given reaction conditions. Option C is the product of a Friedel-Crafts alkylation followed by an elimination reaction, which would also not occur under the given reaction conditions. Option D is the product of a Friedel-Crafts acylation followed by a hydrolysis reaction, which would also not occur under the given reaction conditions.