d
Let $I$ represents the intensity of sound.
Loudness of sound $\quad L=10 \log _{10} \frac{I}{I_{o}}$
$\Longrightarrow L_{2}-L_{1}=10 \log _{10} \frac{I_{2}}{I_{1}}$
$90-40=10 \log _{10} \frac{I_{2}}{I_{1}}$
$5=\log _{10} \frac{I_{2}}{I_{1}} \Longrightarrow \frac{I_{2}}{I_{1}}=10^{5}$