Consider the following statements in the context of human eye :

(a) The diameter of the eye ball is about $2.3\text{ cm}$.

(b) Iris is a dark muscular diaphragm that controls the size of the pupil.

(c) Most of the refraction for the light rays entering the eye occurs at the crystalline lens.

(d) While focusing on the objects at different distances the distance between the crystalline lens and the retina is adjusted by ciliary muscles.

The correct statements are -

According to the NCERT textbook:

• Statement (a) is correct: 'The eyeball is approximately spherical in shape with a diameter of about $2.3\text{ cm}$.'
• Statement (b) is correct: 'Iris is a dark muscular diaphragm that controls the size of the pupil.'
• Statement (c) is incorrect: 'Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea.' The crystalline lens merely provides the finer adjustment of focal length.
• Statement (d) is incorrect: The ciliary muscles change the curvature (and thus the focal length) of the eye lens to focus objects at different distances, while the distance between the lens and the retina remains constant.

Therefore, only statements (a) and (b) are correct.