The correct option is D that
The main function of a conjunction is to connect or combine two clauses/sentences as one. In the given sentence, we can identify the clauses as "She was told" and "she must agree", as they're grammatically complete, with the subject as "She" and the predicate "told" and "agree", a clause is complete with a subject and predicate. The subject is 'whom the sentence is about' and the predicate tells something about the subject. The word that's been left out is 'that', as it does not form a part of any clause, and connects them both. Hence, 'that' is a conjunction and option C is correct. 'She' is a third person singular pronoun. Pronouns modify nouns that have been identified with or mentioned before, thus eliminating the repetition of the noun. It does not link the clauses, hence it's not a conjunction and option A is incorrect. 'Must' is a modal auxiliary. Auxiliary verbs determine the tense or mood (here it's obligation of agreeing) of the sentence. It does not link the clauses together hence it's not a conjunction and option B is incorrect. 'Agree' is a verb that means conforming to or having the same opinions as that of another person. It does not link the clauses, hence it's not a conjunction and option D is incorrect.