The correct answer is C. by.
The preposition “by” is used to indicate a deadline or a point in time by which something must be done. In this case, Ram must be back home by 10 o’clock. This means that he must arrive home no later than 10 o’clock.
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.