**Core Concept**
The ciliary muscle plays a crucial role in the accommodation reflex, which enables the eye to focus on objects at varying distances. **Contraction of the ciliary muscle** relaxes the tension in the suspensory ligaments (zonules) surrounding the lens, allowing it to become more spherical and increase its refractive power. This process is essential for near vision.

**Why the Correct Answer is Right**
When the ciliary muscle contracts, it releases the zonular fibers, which are attached to the lens capsule. This relaxation allows the lens to become more rounded due to the elastic properties of the lens capsule. As the lens becomes more spherical, its refractive power increases, enabling the eye to focus on near objects. This mechanism is mediated by the parasympathetic nervous system, which stimulates the ciliary muscle to contract via the oculomotor nerve (cranial nerve III).

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not related to the action of the ciliary muscle. The iris is responsible for controlling the amount of light entering the eye through the pupil.
* **Option B:** The rectus muscles are responsible for rotating the eye, not controlling accommodation.
* **Option D:** The superior oblique muscle is one of the extraocular muscles responsible for intorsion and depression of the eye, not accommodation.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the ciliary muscle is responsible for the accommodation reflex, which enables the eye to focus on near objects. This process is essential for clear vision in low-light conditions and for reading or other near-vision tasks.

**Correct Answer: C. Contraction of the ciliary muscle relaxes the tension in the suspensory ligaments (zonules) surrounding the lens, allowing it to become more spherical and increase its refractive power.**