If a sum of money double itself in 8 years, then the interest rate in percentage is ?

Answer: Option D

Money is double itself means interest is equal to money.
$$\begin{array}{rl}& \text{P = }\frac{\text{P}\times \text{8}\times \text{r}}{100}\\ & \Rightarrow \text{r = }\frac{100}{8}\\ & \Rightarrow \text{r = 12}\frac{1}{2}\%\end{array}$$