Answer
$3.1\times 10^{-21}\;\rm N\cdot m$
Work Step by Step
We know that the torque exerted by an electric field on a dipole is given by
$$\tau=pE\sin\theta$$
and hence, the maximum torque occurs when $\theta=90^\circ\rightarrow\sin\theta=1$
$$\tau_{max}=pE$$
Plugging the given;
$$\tau_{max}=(6.2 \times 10^{-30})(5\times 10^8)$$
$$\tau_{max}=\color{red}{\bf 3.1\times 10^{-21}}\;\rm N\cdot m$$
