Presbyopia is a visual defect that occurs as a natural part of the aging process. It is caused by the weakening of the ciliary muscles in the eye.

Option 1: Elongation of the eye ball - This is not the cause of presbyopia. Presbyopia is not related to the shape or size of the eyeball.

Option 2: Shortened curvature of the eye lens - This is not the cause of presbyopia. Presbyopia occurs due to changes in the eye`s ability to focus, not changes in the shape of the lens.

Option 3: Weakening of the ciliary muscles - This is the correct answer. The ciliary muscles play a key role in adjusting the shape of the lens to focus on objects at different distances. As we age, these muscles become weaker, making it harder for the lens to change shape and focus on nearby objects.

Option 4: Gradually increasing flexibility of the eye lens - This is not the cause of presbyopia. In fact, presbyopia is the result of the lens becoming less flexible and losing its ability to accommodate or focus on nearby objects.

In conclusion, presbyopia is caused by the weakening of the ciliary muscles in the eye, which leads to