The correct answer is: C. ourselves
The sentence “They agreed amongst themselves that they would not tell anyone” is in the plural form. This means that the subject of the sentence is a group of people. The pronoun “ourselves” is the only plural pronoun that is grammatically correct in this context. The pronouns “himself” and “yourself” are singular pronouns, and the pronoun “themselves” is a reflexive pronoun that is used to refer back to the subject of the sentence. In this case, the subject of the sentence is a group of people, so the reflexive pronoun “themselves” is not grammatically correct.
The pronoun “ourselves” is a plural pronoun that is used to refer to the speaker and one or more other people. In this case, the speaker is a member of the group of people who agreed not to tell anyone. Therefore, the pronoun “ourselves” is the only grammatically correct option.