- The power of a lens is measured in **Diopter (Option 3)**.
- This is the correct answer. ✅
- Diopter is a unit that measures the refractive power of a lens.

- Ampere (Option 1)
- This is a unit of electric current, not related to lenses.

- Candela (Option 2)
- This measures luminous intensity, not the power of a lens.

- Kelvin (Option 4)
- This is a unit of temperature, irrelevant to lens power discussion.

**YOUR ANSWER IS RIGHT**