An electric motor exerts a constant torque $5Nm$ on a flywheel by which it is rotated at the rate of $420rpm$. The power of motor is:

The correct option is D $220watt$
The electric motor rotates at the rate
$420rpm=420\times \frac{2\pi }{60}rad{s}^{-1}$
Thus we get power of motor:
$P=\tau \times \omega$
$=5\times (420\times \frac{2\pi }{60})=220W$