## Core Concept
The power of a lens is defined as the reciprocal of its focal length, measured in meters. The formula to calculate the power of a lens is (P = frac{1}{f}), where (P) is the power in diopters (D) and (f) is the focal length in meters. This relationship is fundamental in ophthalmology and optometry for characterizing lenses used in corrective eyewear.

## Why the Correct Answer is Right
Given that the focal length (f = 0.25) meters, we can substitute this value into the formula to find the power: (P = frac{1}{0.25} = 4) diopters. This means a lens with a focal length of 0.25 meters has a power of 4 diopters. The correct answer reflects this calculation.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not match the calculation of power based on the given focal length.
- **Option B:** This option is incorrect for the same reason as Option A; it does not align with the calculated power of 4 diopters.
- **Option D:** This option suggests a negative power, which would be correct for a diverging lens but not for the calculation provided for a focal length of 0.25 meters, which implies a converging lens.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the power of a lens is inversely proportional to its focal length. This relationship is crucial for understanding how lenses correct vision problems. For example, a lens with a shorter focal length has more power and can correct more severe vision problems.

## Correct Answer Line
**Correct Answer: C. 4**