The POWER of a lens (in diopters, D) is defined as

1. syllabus defines lens power. 2. $P = 1/f$, where $f$ is the focal length expressed in METRES. 3. SI unit: diopter (D), defined as $1\,\text{D} = 1\,\text{m}^{-1}$. 4. Higher power = shorter focal length = stronger converging (or diverging) action. 5. Sign: positive $P$ for converging lens (converging combination), negative $P$ for diverging. 6. Example: a lens with $f = +50\,\text{cm} = +0.5\,\text{m}$ has $P = +2\,\text{D}$. A diverging lens with $f = -25\,\text{cm}$ has $P = -4\,\text{D}$. 7. Options A and C have wrong forms. Option D uses cm instead of m, which gives a number 100× too large. _Source: syllabus Physics Part 2, Ch 9, §9.6.3 (Power of a Lens, Eq. 9.21), p. 18._