Answer : Option A

Explanation :

Let the number = 1
Then, ideally he should have multiplied 1 by 5/3.
Hence the correct result was 1 x (5/3) = (5/3)
By mistake, he multiplied 1 by 3/5.
Hence the result with the error = 1 x (3/5) = (3/5)

$MF#%\begin{align}
&\text{Error = }\dfrac{5}{3} - \dfrac{3}{5} = \dfrac{25-9}{15} = \dfrac{16}{15}\\
&\text{percentage error = }\dfrac{\text{Error}}{\text{True Value}}\times 100=\dfrac{\left(\dfrac{16}{15}\right)}{\left(\dfrac{5}{3}\right)} \times 100\\
&= \dfrac{16 \times 3 \times 100}{15 \times 5} = \dfrac{16 \times 100}{5\times 5} = 16 \times 4 = 64\%\\
\end{align} $MF#%