The correct option is B though
The main function of a conjunction is to combine or link two clauses/sentences as one. In the given sentence, the clauses are identified as "His dish was the best" and "he cooked the slowest", because they're grammatically complete. The latter is the subordinate/dependent clause as it depends on the former for its full meaning. The word that's left behind is "though" as it performs the sole function of connecting these clauses, and is used for contrast-related sentences, where the meaning of the subordinate clause is the opposite of the main clause. 'Though' forms a relation between the clauses by referring to their contrast meaning. It's a conjunction, hence option B is correct. 'He' is a third person singular pronoun and refers to the person being spoken about. Pronouns modify nouns that have been identified with or mentioned before, thus eliminating the repetition of the noun. It does not connect the clauses, hence option A is incorrect. 'Cooked' is a verb of the past tense, that refers to the act of preparing a meal. It does not connect the clauses, hence option C is incorrect. 'Best' is an adjective that refers to something of the highest quality or degree. Here it modifies the noun 'dish' by expressing it as a dish of the highest quality of being 'good'. It does not link the sentences, hence it's not a conjunction and option D is incorrect.