Answer : Option A

Explanation :

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Solution 1
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Let the sum of money be Rs.x
After 40 years, this becomes 3x
Simple Interest = 3x - x = 2x
$MF#%\begin{align}&\text{Simple Interest = }\dfrac{\text{PRT}}{100}\\\\&2x =\dfrac{x \times \text{R} \times 40}{100}\\\\&2 =\dfrac{\text{R} \times 40}{100}\\\\&200 =40\text{R}\\\\&\text{R} = 5\%\end{align}$MF#%
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Solution 2
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If a sum of money becomes n times in T years at simple interest, then the rate of interest per annum can be given be
$MF#%\text{R = }\dfrac{100(\text{n}-1)}{\text{T}}\%$MF#%