**Core Concept**
The question tests the understanding of corrective lenses used to treat astigmatism, a common vision disorder caused by irregular curvature of the cornea or lens inside the eye.

**Why the Correct Answer is Right**
Astigmatism occurs due to irregular curvature of the cornea or lens, resulting in blurred vision at all distances. To correct astigmatism, a cylindrical lens is used, which is designed to refract light in one meridian more than the other. This is achieved by adding a cylindrical power to the spherical power of the lens, with the axis of the cylinder aligned with the axis of the astigmatism.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely a distractor, as toric lenses are not typically used to treat astigmatism. Toric lenses are actually used to treat anisometropia, where the difference in refractive power between the two eyes is more than 1.0 diopter.
* **Option B:** This option is incorrect because spherical lenses are used to correct spherical refractive errors, such as myopia and hyperopia, but not astigmatism.
* **Option C:** This option is not a valid choice, as it is incomplete.

**Clinical Pearl / High-Yield Fact**
A toric lens can be identified by the presence of markings or etchings on the lens, which indicate the axis of the cylinder.

**Correct Answer: C. Cylindrical lens. Cylindrical lens is used to treat astigmatism.**