c
When the ball attain terminal velocity
$F _r=\left( mg - F _{ B }\right)(\because a =0)$
$= V \sigma_{ b } g - V \rho_{\ell} g$
$= Vg \left(\sigma_{ b }-\rho_{\ell}\right)$
$=\frac{4}{3} \pi\left(10^{-3}\right)^3 \times 9.8(10.5-1.5) \times 10^3$
$=3696 \times 10^{-7}\,N$
So, $x=7$