Is you mother working in ______ old office building.
Is you mother working in ______ old office building.
Explanation
"Old" starts with a vowel sound, so we use "an" before it.
Correct sentence: "Is your mother working in an old office building?"
"Old" starts with a vowel sound, so we use "an" before it.
Correct sentence: "Is your mother working in an old office building?"