In the given figure, by what angle should the mirror be rotated such that the reflected ray becomes horizontal along positive x - axis?

The correct option is C ${10}^{\circ }$ clockwise
Given ,
$\angle i=\angle r={70}^{\circ }$

For the RR to become horizontal, it should further rotate by ${20}^{\circ }$ in clockwise sense.

If mirror rotates by $\theta$ , then $RR$ rotates by $2\theta$ in the same sense.

So , mirror should be rotated by ${10}^{\circ }\circlearrowright$


Hence, option (c) is the correct answer.

$\begin{array}{c}\text{Note-1 :- The angle turned by the reflected ray is twice the angle turned by the mirror.}\\ \text{Note-2 :- If mirror rotates clockwise, then reflected ray will also rotate clockwise.}\end{array}$