[أ/ أحمد البسيونى]

Choose Which one is correct? My office (where - which) I'm using at the moment had only one classroom.
Where are you getting such a sentence? The office you're using had a classroom? But it doesn't now? The different verb tenses are confusing, as is the idea of your office (which sounds like one room from the rest of the sentence) having (had) a classroom.
The correct relative pronoun here is which. You use a thing. You are using the office.