The correct option is C since
The main function of a conjunction is to join or connect two clauses as one. In the given sentence, the comma "," makes it easier to separate the clauses as "you are taller than me" and "you should be the one to climb the wall", where the former is the subordinate clause that depends on the latter, the main clause, for its full meaning. The sentence is cause-related, where the subordinate clause is the cause for which the action in the main clause takes place "you should climb". "Since" does the job of linking the clauses, it performs no other function as another part of speech, hence 'since' is a conjunction and option B is correct. 'You' is a second person singular pronoun, pronouns modify nouns that have been identified with or mentioned before. Here, 'you' modifies the listener who's being addressed. It does not link the clauses together, as it's a part of the main clause. Hence option A is incorrect. 'Should' is a modal auxiliary, a verb that determines the tense or mood of the sentence, here 'should' refers to an obligation to "climb the wall". It does not link the clauses, it's a part of the main clause, hence it's not a conjunction and option C is incorrect. 'Wall' is a noun which means "a structure that is built for security reasons', it does not link the clauses together as it's a noun. Hence option D is also incorrect.