Ram must be back home . . . . . . . . 10 o’clock. A. for B. to C. by D. till

The preposition “for” is used to indicate a period of time during which something happens. For example, “Ram went to the store for an hour.”

The preposition “to” is used to indicate a destination or goal. For example, “Ram went to the store to buy some milk.”

The preposition “till” is used to indicate a point in time until which something happens. For example, “Ram stayed up till midnight.”

In this case, the preposition “by” is the most appropriate choice because it indicates a deadline or a point in time by which something must be done.