He works in .... old office building.

Answer: Both "an" and "the" are acceptable depending on context.

Explanation: Choose the article based on whether the building is being referred to specifically or not.

• Use "an" for a non-specific (any) old office building. Example: He works in an old office building.

• Use "the" for a specific, known old office building. Example: He works in the old office building.

Note: "A" is incorrect before "old" because "old" begins with a vowel sound, so the correct indefinite article is "an".