1. Key idea: The focal length of a mirror depends only on its radius of curvature: $f = \frac{R}{2}$ for a spherical concave mirror.

2. What changes when dipped in a liquid?
   For a lens, focal length depends on refractive index of the surrounding medium.
   But for a mirror, image formation occurs due to reflection, and the law of reflection is: $i = r$ This law is independent of the refractive index of the surrounding medium.

3. Therefore, dipping the concave mirror in a liquid does not change its radius of curvature or its reflecting properties.

4. Hence the focal length remains unchanged: $f' = \frac{R}{2} = f$

5. Checking options:

   • A: $\mu f$ ❌
   • B: $f$ ✅
   • C: $\dfrac{f}{\mu}$ ❌
   • D: $\dfrac{f}{\mu-1}$ ❌

Therefore, the correct answer is B.