$2\left(\frac{v_1}{2 l_1}\right)=\frac{v_2}{4 l_2}$ $\therefore \quad \frac{\sqrt{T / \mu}}{l_1}=\frac{320}{4 l_2}$
( $\mu=$ mass per unit length of wire)
or $\frac{\sqrt{50 / \mu}}{0.5}=\frac{320}{4 \times 0.8}$
Solving we get $\mu=0.02 \frac{\mathrm{kg}}{\mathrm{m}}=20 \frac{\mathrm{g}}{\mathrm{m}}$
$\therefore$ Mass of string

=\left(20 \frac{\mathrm{g}}{\mathrm{m}}\right)(0.5 \mathrm{~m})=10 \mathrm{~g}

$\therefore$ The correct option is (b).