I haven’t seen her in . . . . . . . . nine years. A. a B. an C. the D. no article

The correct answer is: no article.

The sentence “I haven’t seen her in nine years” is about a specific person, so we do not need to use an article. If we were talking about a general person, we would use “a” or “an”:

• I haven’t
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  seen a woman in nine years.
• I haven’t seen an old man in nine years.

But in this case, we are talking about a specific person, so we do not need to use an article.