Method for changing included angle between incident light and reflected light by a certain angle

According to the subject conditions: the included angle between incident light and reflected light is 120, and the incident angle is 60.

In order to make the included angle between the reflected light and the incident light become 100, and the reflected angle is equal to the incident angle, then the incident angle should become 50, so that the incident light will not move, and the mirror can rotate counterclockwise10, so option D meets.

So choose D.