Dioptre is the unit of:
 A. telescope
 B. microscope
 C. power of lens
 D. eyesight

Hint:The word dioptre is originally derived from the British word ‘diopter’, which is the measuring unit for curved mirrors or for lenses with optical power. From this we can infer that this unit is mainly used in the measurement of power of the spherical mirrors as well as the optical lenses around us. This has been used for measurements as it is proved better than the measurements using focal length.

Complete step by step answer:
The measurement of one dioptre is equivalent to the reciprocal of the focal length. In other words, we can say that 1 dioptre is defined as the measurement unit of the optical power of a lens or a curved mirror, and is equivalent to the focal length of the lens or the mirror.
That is, dioptre is the SI unit of power and is denoted by D
One dioptre (D)= P = (1/f)
Here, f is the focal length measured in metres.
Therefore, option C is the correct choice among the four.

Note: Along with the measurement of power of lens using dioptre, we can also measure the unit of curvature equivalent to the reciprocal of the radius. Optical power is preferably measured using dioptre rather than focal length because the lensmaker equation uses the reciprocal of the focal length and we can directly substitute dioptre to make the calculations. Also when lenses with similar positions are lined up near to each other, its powers get added up.