Induced emf due to a rotating metallic disc in uniform magnetic field $B$ is given by expression,

$e=\frac{1}{2}B\omega a^2$

where, $\omega =$ angular speed and $a=$ radius of the disc.

Given, radius of metallic disc $a=10 \mathrm{cm}$, angular speed of metallic disc, $\omega =200 \mathrm{rad} {\sec }^{-1}$ and magnetic field $B=5 \mathrm{mT}$

Now, putting the values in above expression, $e=\frac{1}{2}B\omega a^2$

$=\frac{1}{2}\times 200\times 5\times {10}^{-3}\times {\left(10\times {10}^{-2}\right)}^2$

$\Rightarrow e=5 \mathrm{mV}$