Defect of Astigmatism:
Astigmatism is a common vision problem that occurs when the cornea or lens of the eye has an irregular shape, causing blurred or distorted vision. In this condition, the eye fails to focus light evenly onto the retina, resulting in both near and distant objects appearing blurry or distorted.

The defect of astigmatism can be rectified by using a cylindrical lens. Here's how it works:
1. Cylindrical Lens:
- A cylindrical lens is a type of lens that has different curvatures in different meridians.
- It has a spherical curvature in one meridian and a cylindrical curvature in the perpendicular meridian.
- The cylindrical lens is designed to correct the uneven curvature of the cornea or lens, which is the main cause of astigmatism.
2. Correcting Irregular Curvature:
- The cylindrical lens is prescribed in such a way that it counteracts the irregular curvature of the cornea or lens.
- The lens is positioned in front of the eye, and its cylindrical power is oriented to align with the axis of astigmatism.
- By doing so, the cylindrical lens helps to focus light properly onto the retina, correcting the blurry or distorted vision caused by astigmatism.
3. Different Power in Different Meridians:
- Unlike a convex or concave lens that has the same power in all meridians, a cylindrical lens has different powers in different meridians.
- It compensates for the difference in refraction between the two principal meridians of the eye, thus correcting astigmatism.
4. Additional Correction:
- In some cases, astigmatism may be accompanied by other refractive errors like nearsightedness (myopia) or farsightedness (hyperopia).
- In such situations, a combination of a cylindrical lens and a spherical lens may be prescribed, known as a bifocal lens.
- The bifocal lens corrects both astigmatism and the additional refractive errors, providing clear vision at different distances.
Therefore, the correct answer to rectify the defect of astigmatism is a cylindrical lens (Option B). It helps to compensate for the irregular curvature of the cornea or lens, allowing light to focus properly on the retina and improving vision clarity.

B) cylindrical lens