Correct answer is 'C'
The muscle of iris control the size of pupil.

The correct answer is option 'C' - Opening of the pupil.

The iris is a structure located in the eye that is responsible for controlling the amount of light that enters the eye through the pupil. It is a circular, pigmented muscle that surrounds the pupil, which is the black opening in the center of the iris. The iris controls the size of the pupil by contracting or relaxing its muscles, thereby regulating the amount of light that reaches the retina at the back of the eye.

The muscles of the iris can be divided into two sets: the sphincter pupillae and the dilator pupillae. These muscles work in opposition to each other to adjust the size of the pupil.

1. Sphincter pupillae: This circular muscle is located in the iris and encircles the pupil. When this muscle contracts, it causes the pupil to constrict, becoming smaller in size. This constriction of the pupil is called miosis. The sphincter pupillae is innervated by the parasympathetic nervous system, specifically the oculomotor nerve (cranial nerve III). Constriction of the pupil occurs in response to bright light or during close vision tasks, such as reading.

2. Dilator pupillae: This radial muscle is located in the iris and extends outward from the center towards the edge of the iris. When this muscle contracts, it causes the pupil to dilate, becoming larger in size. This dilation of the pupil is called mydriasis. The dilator pupillae is innervated by the sympathetic nervous system, specifically the superior cervical ganglion. Dilation of the pupil occurs in response to dim light or during tasks that require increased visual sensitivity, such as night vision.

The opening of the pupil is crucial for regulating the amount of light that enters the eye. In bright conditions, the sphincter pupillae contracts, reducing the size of the pupil to limit the amount of light entering the eye. This prevents excess light from overwhelming the retina and causing discomfort or glare. In dim conditions, the dilator pupillae contracts, enlarging the size of the pupil to allow more light to enter the eye and improve visual sensitivity.

In conclusion, the muscles of the iris control the opening of the pupil, determining the amount of light that enters the eye and regulating visual sensitivity.