MEANS 'exactly at a particular time'.Eg. Fireworks started at the stroke of ten.From the explanation and example that are given above, it can be understood that 'at/on' should be used in the given blank.Therefore, option 1 is the correct answer. Complete SENTENCE: We got back to the hotel at the stroke of midnight. LET us LEARN the usages of other prepositions:with: in the company or presence of somebody/something.Eg. I have a client with me right now.up: towards or in a higher position.Eg. He JUMPED up from his chair.in: at a point within an area or space.Eg. I read about it in the paper.