A reflexive pronoun ends in ..self or ...selves and refers to another noun or pronoun in the sentence (yourself, himself, myself, itself etc).
The noun used in the sentence is 'monkey' whose reflexive pronoun is 'it' as it is an animal whose gender is not specified. The corresponding reflexive pronoun for 'it' is 'itself'. Therefore, the correct answer to fill in the blank is option B) itself..
If the pronoun used in the sentence would have been 'her', then the corresponding reflexive pronoun would be option A) herself.
If the pronoun used in the sentence would have been 'they', or the noun used would have indicated a group of people, then the corresponding reflexive pronoun would be option C) themselves.
If the pronoun used in the sentence would have been 'I', then the corresponding reflexive pronoun would be option D) myself.
So, option A,C and D are incorrect.