The answer to your question would be that the sentence that contains a problem with pronoun agreement is the following one: When somebody tells you they will help you, you should believe them. That is, your answer would be A.
"Somebody" is singular and then "they" is plural. This sentence is an example of the use of "singular they". Singular they is the use of "they" and any of its derivative forms (them, their, theirs, and themselves) as a singular pronoun. It typically occurs with an unspecified antecedent, just as the sentence in question (somebody).