Correct option is 10g
Explanation:
Given, 2 nd harmonic of $\mathrm{I}=$ Fundamental of II
$
\begin{array}{l}
\therefore \quad 2\left(\frac{v_{1}}{2 l_{1}}\right)=\frac{v_{2}}{4 l_{2}} \Rightarrow \frac{T / \mu}{l_{1}}=\frac{v_{2}}{4 l_{2}} \\
\Rightarrow \quad \mu=\frac{16 T l_{2}^{2}}{v_{2}^{2} l_{1}^{2}}=\frac{16 \times 50 \times(0.8)^{2}}{(320)^{2} \times(0.5)^{2}} \\
\quad=0.02 \mathrm{~kg} / \mathrm{m}
\end{array}
$
$
\therefore \quad m_{1}=\mu l_{1}=(0.02)(0.2)
$
$
=0.01 k g=10 g
$