The correct option is D anybody
The sentence requires an indefinite pronoun to complete its meaning. Indefinite pronouns do not identify people or things specifically but in general and non-specific terms. They include words like everyone, everybody, everything, no-one, nobody, nothing, anyone, anybody, anything, someone and so on. This rules out usage of Options A and C as they are not indefinite pronouns but nouns.
The usage of ‘is’ indicates that the indefinite pronoun should be singular. The usage of ‘not’ in ‘isn’t’ indicates that the indefinite pronoun should be the one that could be used in a negative sentence. This rules out usage of Option B as ‘somebody’ is used in a positive sentence. Hence, Option D is the correct answer.