Explanation:

- To find out how many years it will take for the sum to double, we can use the Rule of 72. This rule states that you can divide 72 by the annual interest rate to get a rough estimate of the years needed to double the investment.

- In this case, the interest rate is 26% per annum.

- Divide 72 by 26, which results in approximately 2.77 years.

- Thus, the investment will double in around 2.77 years. This rounds to 3 years when considering the listed options.

Option 2: 3