In the given sentence the subject and the object of the verb is 'they'.
'Himself' and 'Themselves' are the reflexive pronouns for the singular and plural third persons respectively. As in the given sentence a plural noun is used, the reflexive pronoun 'themselves' must be the answer.
Thus option C is the correct answer and option A is incorrect.
The reflexive pronoun for singular second person is 'yourself'. Thus option B is incorrect.
The reflexive pronoun of 'one' is 'oneself'. Thus option D is also incorrect.