The correct option is A Since
A clause is said to be subordinate when it depends on another clause (principal clause) for its full meaning. The subordinate clause (we will serve him a chocolate mousse) is the effect of the principal clause (He has a preference for chocolate) which is a cause. When a clause is a reason or cause, we use the conjunctions 'because', 'since' and so on. The clause here is a reason for the other part of the sentence, because he preferred chocolate, he was served a chocolate dessert, hence option B is used, it's correct. 'Though' is a conjunction which combines sentences or clauses that are contrast in meaning. Since none of the clauses here are contrast in their meaning, in fact one is the cause for the other, option D is incorrect. 'But' is a coordinating conjunction used when two sentences that are combined are independent on their own and contrast in meaning, since one of them here is dependent on the other and are not contrast in meaning, 'but' cannot be used. Hence option A is incorrect. 'After' is a conjunction used for time-related sentences. Since the given clauses are not time-related, option C is incorrect.