The correct answer is A) Seven years.
Although is not a strict time limit for ratifying an amendment, seven years is generally accepted as reasonable time for ratification.
Since the 18th Amendment, Congress considers that an amendment could be inoperative unless it is ratified in a period of seven years. So, although Congress does not set a limit of time to ratify the amendment, members of the Congress agree that a seven-year period is the ideal time to do so.
The other options of the question were, b) 10 years, c) 16 years, d) 21 years.