A

Step 1: Understand the relationship between the focal length of a mirror and the medium it is in.
Step 2: The focal length (f) of a mirror is given by the formula:
f = \frac{R}{2}, where R is the radius of curvature.
Step 3: In a spherical mirror, when the medium changes (for example, from air to water), the effective focal length changes based on the refractive indices of the media.
Step 4: The formula for the focal length in a different medium is modified due to the refractive index (n).
Step 5: The new focal length (f') when submerged in water is given by:
f' = \frac{f}{n}, where n is the ratio of the refractive indices of air to water.
Step 6: For air, n is approximately 1, and for water, it's around 1.33, thus:
f' = \frac{f}{1.33}
Step 7: This indicates that the focal length becomes shorter when in water.
Therefore, the focal length of the mirror decreases when dipped in water.