C

Explanation:
Focal power of a lens is a measure of the lens's ability to converge or diverge light. It is defined as the reciprocal of the focal length (in meters) of the lens. The formula for focal power (P) is given by:
$$ P = \frac{1}{f} $$
where
- $P$ is the focal power in diopters (D)
- $f$ is the focal length in meters

The unit 'diopter' is specifically used for this purpose, indicating how strongly a lens converges or diverges light. Other units like Watt (for power), Horse power (for mechanical power), and Lux (for illuminance) are not applicable to focal power.

Therefore, the correct answer is Dioptre (Option C).