The focal length of the concave mirror in water will be 3/4f. Hence the correct option is c.
When a concave mirror is immersed in water, the change in medium affects the focal length due to the variation in the refractive index. In this case, the refractive index of water (μ= 4/3) is greater than that of air (μ=1).
The focal length of a mirror in a different medium is given by the lens-maker's formula: /f1=(μ−1)( 1/R1-1/R2), where f is the focal length, μ is the refractive index, and R1 and R2 are the radii of curvature of the mirror.
As water has a refractive index of 4/3, the lens-maker's formula, when applied to the concave mirror in water, results in a focal length f water = 3/4f. Therefore, the focal length of the concave mirror immersed in water is 3/4 times the focal length in air. Hence the correct option is c.