The focal length of 0.25 m power of the lens is
It is measured as reciprocal of the focal length in metres i.e. P = 1/f .  The unit of power is dioptre (D). One dioptre is the power of a lens of focal length one metre.

The focal length (f) of a lens is the distance between the optical centre and the principal focus.
Power of a lens (P) is defined as the ability of the lens to converge a beam of light falling on the lens.
40 D
4 D
1/4 D
25 D