Long-sight defect, also called hyperopia, means a person can see far objects clearly but has trouble seeing nearby objects. This happens because the eye focuses light behind the retina instead of on it.

To fix this, a convex lens is used. It is thicker in the middle and thinner at the edges. This lens bends light rays inward to focus correctly on the retina, helping the person see close objects clearly.

The other lenses:
- Diverging lens (concave lens) is used to fix short-sight, not long-sight.
- Concave lens spreads out light and helps short-sighted people.
- None of these is incorrect here.

So, the correct answer is the convex lens, which corrects long-sight or hyperopia.