1. Set up the geometry

Let the angle between the two plane mirrors $M_1$ and $M_2$ be $\theta$.

The incident ray is:

• incident on $M_1$
• parallel to $M_2$

After reflection from $M_2$, the final ray is:

• parallel to $M_1$

2. Use the property of successive reflections

When a ray is reflected successively from two plane mirrors inclined at an angle $\theta$, the net deviation in the direction of the ray is:

$2\theta$

That is, the angle between the initial direction and the final direction is $2\theta$.

3. Find the angle between initial and final rays

Initially, the ray is parallel to $M_2$. Finally, the ray is parallel to $M_1$.

Therefore, the angle between the initial and final directions is exactly the angle between the mirrors:

$\theta$

But from the two-reflection result, this angle must also equal:

$2\theta$

This seems contradictory unless we interpret the directions carefully.

Since the ray changes side after reflection, the actual angle between the directed initial and final rays is the supplement of $\theta$:

$180^\circ - \theta$

Hence,

$2\theta = 180^\circ - \theta$

$3\theta = 180^\circ$

$\theta = 60^\circ$

4. Check with options

• A: $45^\circ$
• B: $60^\circ$ ✅
• C: $75^\circ$
• D: $90^\circ$

So the correct answer is:

$\boxed{60^\circ}$

Therefore, Option B is correct.