That monkey is scratching . . . . . . . .

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.