Answer
The angle between $i$ and $r'$ is $~~180^{\circ}$
Work Step by Step
Note that the angle of a reflected ray is equal to the angle of an incident ray.
When the ray reflects off the vertical surface, the reflected ray has an angle of $\theta$
The incident ray on the horizontal surface has an incident angle of $90^{\circ}-\theta$, and the reflected ray also has an angle of $90^{\circ}-\theta$
We can add the four angles to find the angle between $i$ and $r'$:
$\theta+\theta+(90^{\circ}-\theta)+(90^{\circ}-\theta) = 180^{\circ}$
The angle between $i$ and $r'$ is $~~180^{\circ}$.
