Presbyopia is a visual defect that commonly occurs as people age. It is characterized by the inability to focus on nearby objects or small print. This condition is caused by the weakening of the ciliary muscles in the eye.

The ciliary muscles are responsible for changing the shape of the lens in the eye, allowing it to focus on objects at varying distances. When the ciliary muscles weaken with age, they become less efficient in adjusting the lens, leading to difficulties in focusing on nearby objects.

Here is a detailed explanation of each option and why option 'C' is the correct answer:

a) Elongation of the eye ball: This option is not the correct answer because elongation of the eyeball is associated with another condition called myopia or nearsightedness. In myopia, the eyeball becomes elongated, causing light to focus in front of the retina instead of directly on it. This results in blurry vision for distant objects.

b) Shortened curvature of the eye lens: This option is also not the correct answer. The curvature of the lens plays a role in focusing light onto the retina, but presbyopia is not caused by a shortened curvature of the lens.

c) Weakening of the ciliary muscles: This is the correct answer. The ciliary muscles are responsible for controlling the shape of the lens in the eye. As we age, these muscles naturally weaken, leading to a decreased ability to focus on nearby objects.

d) Gradually increasing flexibility of the eye lens: This option is not the correct answer. While the flexibility of the lens does change with age, it does not result in presbyopia. Presbyopia is primarily caused by the weakening of the ciliary muscles, not the flexibility of the lens itself.

In conclusion, presbyopia is a visual defect caused by the weakening of the ciliary muscles in the eye. These muscles are responsible for adjusting the lens to focus on nearby objects. As they weaken with age, people experience difficulty in focusing on close-up objects, leading to the need for reading glasses or other corrective measures.