Answer: B — Concave lens
Elongation of the eyeball causes myopia (near-sightedness). It is corrected by using a concave lens of suitable power.
Source: Chapter 10, Section 10.2(a) – Myopia
The key link is: elongated eyeball → myopia → concave lens. The passage explicitly states myopia "may arise due to…elongation of the eyeball" and "can be corrected by using a concave lens of suitable power." Students often confuse this with convex lens (used for hypermetropia). Remember: Myopia = concave; Hypermetropia = convex.