c
Loudness of sound is given by

\(\mathrm{dB}=10 \log \frac{I}{\mathrm{I}_{0}}\) (\(I\) is intensity of sound, \(I_0\) is reference intensity of sound)

\(120=10 \log \left(\frac{I}{I_{0}}\right)\)

\(\Rightarrow I=1 \mathrm{W} / \mathrm{m}^{2}\)

Also, \(\mathrm{I}=\frac{\mathrm{P}}{4 \pi \mathrm{r}^{2}}=\frac{2}{4 \pi \mathrm{r}^{2}}\)

\(\therefore r=\sqrt{\frac{2}{4 \pi}}=\sqrt{\frac{I}{2 \pi}} \mathrm{m}=0.399 \mathrm{m}\)

\(\approx 40 \mathrm{cm}\)