At what rate percent will a sum of double itself in 12 years ?

Correct option is B

Given:

Let the principal amount be 'P' and the time be 12 years.

Formula Used:
For a sum to double, we can use the formula for simple interest:
​$$A = P(1 + \frac{R \times T}{100})$$​​
Since the sum doubles, A = 2P, and T = 12 years.

​$$2P = P(1 + \frac{R \times 12}{100})\\\ \\2 = 1 + \frac{12R}{100}\\\ \\2 - 1 = \frac{12R}{100}\\\ \\1 = \frac{12R}{100}\\\ \\R = \frac{100}{12} = 8.33$$​​

Answer: The rate percent is $$8 \frac{1}{3} \%$$ =  8.33% per annum.