Explanation:

The correct answer to the given question is option 'D', which states that the eye is not able to decrease the focal length beyond a limit. Let's understand why this is the correct answer.

Focal Length of the Eye:
- The focal length of an eye is the distance between the eye's lens and the retina when the eye is focused on an object at infinity.
- In a normal eye, the focal length is approximately 2.5 cm.
- This means that when the eye is relaxed and focused on a distant object, the distance between the lens and the retina is approximately 2.5 cm.

Accommodation of the Eye:
- When we need to see objects at different distances, the eye can adjust its focal length through a process called accommodation.
- Accommodation is the ability of the eye to change the shape of its lens in order to focus on objects at different distances.
- When we look at objects closer than infinity, the ciliary muscles of the eye contract and the lens becomes thicker.
- By changing the shape of the lens, the eye increases its refractive power and decreases its focal length to focus on nearby objects.

Limitations of Accommodation:
- However, there are limitations to the accommodation ability of the eye.
- As objects are brought closer to the eye, the ciliary muscles contract even more to make the lens thicker.
- At a certain point, the ciliary muscles reach their maximum contraction limit, and the lens cannot become any thicker.
- This limits the eye's ability to further decrease the focal length and focus on objects closer than a certain distance.
- In a normal eye, this limit is typically around 25 cm.

Explanation of Incorrect Options:
- Option 'A' states that the focal length of the eye is 25 cm. This is incorrect because the focal length of a normal eye is approximately 2.5 cm, not 25 cm.
- Option 'B' states that the distance of the retina from the eye-lens is 25 cm. This is incorrect because the distance between the lens and the retina is not fixed and can change based on the eye's accommodative power.
- Option 'C' states that the eye is not able to decrease the distance between the eye-lens and the retina beyond a limit. This is incorrect because the distance between the lens and the retina can decrease as the eye accommodates to focus on closer objects. It is the focal length that cannot be decreased beyond a certain limit.

Therefore, option 'D' is the correct answer as it correctly explains that the eye is not able to decrease the focal length beyond a limit, which limits its ability to focus on objects closer than a certain distance.