Answer: (a) A convex lens has 4 dioptre power having a focal length 0.25 m
1 dioptre of power of a lens is described as the unit of measurement of the optical power of a lens or curved mirror equivalent to the reciprocal of focal length (f). Measurement of the focal length in a meter.
1 dioptre (D) = P = 1/f
Where f is focal length
P = 1/ 0.25
P = 4
Positive value for focal length indicates a convex lens.