A normal eye is not able to see object closer than $25\text{cm}$ because,

The correct option is D the eye is not able to decrease the focal length beyond a limit.
When the eye is focused on a closer object, the ciliary muscles are strained and the focal length of the eye-lens decreases. Because of some reason, the eye is not able to decrease the focal length beyond a limit, hence a normal eye is not able to see objects closer than $25\text{cm}$.