Concept:Prepositions of time indicate the specific relationship between an action and time.
Explanation:The sentence states that the operation was completed between nine and ten o'clock.
This describes a time interval with a starting point and an ending point.
The preposition
Between is used to refer to a period of time that lies inside two definite points.
The hours of nine and ten form such a range.
Options A, C, and D do not express this two-sided time span correctly.
Therefore, the correct choice is
Between.
Answer:B. Between