Fill in: was, wasn't, were or weren't. A: Where you last night, Mark?

1. The question is about a single person (Mark) and refers to the past. Therefore, 'was' is the correct form of the verb 'to be' for the third person singular in the past tense.

2. The sentence structure requires a question word followed by the verb.

3. The correct completion is 'Where were you last night, Mark?'.