Question

the policeman tapped on the window____ and the driver opened it quickly.
, : nothing needed

Explanation:

The sentence has two independent clauses ("The policeman tapped on the window" and "the driver opened it quickly") that are connected without a coordinating conjunction. A comma is not used here to join two independent clauses without a conjunction, and a colon is inappropriate as it does not introduce an explanation, example, or related detail. The two clauses can stand as separate statements, but in this structure, no punctuation is needed in the blank to avoid a comma splice, and the flow works without additional punctuation.

Answer:

nothing needed