The correct answer is D. no article.
The article "a" or "an" is used before singular countable nouns that are not specific. In this case, the noun "five years" is a non-specific amount of time, so it does not need an article.
The article "the" is used before singular countable nouns that are specific. For example, if you said "I haven't seen him in the five years since he moved away," you would be referring to a specific five-year period.
In this case, the speaker is simply saying that they have not seen the person in five years, so there is no need to specify which five years they are referring to.
