Correct Answer: 3%

Solution : The formula for calculating simple interest is given by:
Simple Interest (SI) = $\frac{P×R×T}{100}$, where
$P$ is the principal amount,
$R$ is the rate of interest per annum, and
$T$ is the time in years.
The simple interest received in 20 years = $\frac{3}{5}P$ 
⇒ $\frac{3}{5}×P = \frac{P×R×20}{100}$
⇒ $\frac{3}{5} = \frac{R×20}{100}$
⇒ $R = 3$%
Therefore, the annual rate of interest is $3$%.
Hence, the correct answer is 3%.