Explanation:

- When a sum doubles in 7 years at simple interest, it means the interest earned in 7 years is equal to the principal amount.

- Simple interest is calculated using the formula: SI = P * R * T / 100, where P is the principal, R is the rate, and T is time in years.

- Since the sum doubles, in 7 years, the interest is equal to the principal. Thus, P = P * R * 7 / 100, simplifying to R = 100/7.

- To make the same sum four times, the interest should be three times the principal.

- Using the same formula, 3P = P * R * T / 100. Substitute for R and solve: 3P = P * (100/7) * T / 100, resulting in T = 21 years.

- The correct option is option 3: 21 years.

Option 3: 21 years is correct.