Since the article is "an", the next word needs to be one that starts with a vowel sound. Only A or D could work for that reason.

Jonathan made a good point about the indefinite article "an." The individual who constructed the sentence might prefer D as an answer, but there's no reason why A wouldn't work as well. Why not work an adorable office, too? It sounds like an agreeable idea to me.

3

A/An? a comfortable.....................................

A