Explanation:

- To determine how many years it will take for a sum to become 64 times, given it becomes 4 times in 4 years under compound interest, we need to understand the pattern.

- In 4 years, the sum quadruples. This implies that every 4 years, the amount is multiplied by 4.

- Initially, the sum is \( 1 \times \text{Principal} \).

- After 4 years, it is \( 4 \times \text{Principal} \).

- After another 4 years (total 8 years), it becomes \( 4 \times 4 = 16 \times \text{Principal} \).

- After another 4 years (total 12 years), it becomes \( 16 \times 4 = 64 \times \text{Principal} \).

- Therefore, the sum becomes 64 times the principal in 12 years.

- Option 2: 12 years is the correct answer.

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