Correct Answer:
B. in
The correct preposition to indicate a future point in time within a specified duration is 'in'.
- 'in about twenty minutes' means that after approximately twenty minutes have passed, the action will occur. This is a standard usage for expressing future timeframes.
- 'on' is incorrect as it is typically used for specific days (e.g., on Monday) or dates (e.g., on June 1st), or for surfaces.
- 'at' is used for precise times (e.g., at 3 PM) or specific locations, not for a duration leading up to an event.
- 'over' implies duration (e.g., over the weekend) or position, but not a future point of readiness.
Thus, 'in' is the only option that correctly conveys the intended meaning of readiness within a future time span.