No sooner_______ he_______ us.1)had....seen2) has... seen3) was... seeing4) did... see

The right number is 1: "No sooner had he seen us". Because its Past Perfect. The verb "to be" have only one form "had been" in Past Perfect. We use Past Perfect when we already talking about the past and want to talk about an earlier past time. And we use Past Perfect when we want to show that a past action or situation took place before another past action.