Concept:The correct preposition must show a time period that starts and ends at two given points.Explanation:The phrase “the hours of nine and ten” names two specific times.The preposition “between” is used when something happens in the interval from one time to another.Therefore, the sentence means the surgeons completed the operation sometime from nine o’clock to ten o’clock.The other options do not fit this meaning of an interval.Answer:C. between