Explanation

(c) The angular momentum of a moving particle $J = 4\sqrt{t}\ kg\ m^{2}s^{- 1}$
Torque acting on a moving particle
$\tau = \frac{dJ}{dt} = \frac{d}{dt}\left( 4\sqrt{t} \right) = 4\frac{d\sqrt{t}}{dt} = 4\left( \frac{1}{2}\ t^{\frac{1}{2} - 1} \right)$
$\tau = \frac{4}{2}t^{\frac{- 1}{2}}Nm = \frac{2}{\sqrt{t}}\ Nm$