Concept: This question tests your understanding of the structural and functional differences between rod cells and cone cells, the two types of photoreceptor cells in the human retina.

Why (A) is correct: Rod cells are responsible for twilight (scotopic) vision and detect dim light, while cone cells are responsible for daylight (photopic) vision and detect bright light and color. Rod cells contain the pigment rhodopsin (visual purple), which is a derivative of Vitamin A. Cone cells contain different photopigments (photopsins) that are sensitive to red, green, and blue light, enabling color vision. The statement correctly identifies that rod cells contain rhodopsin and cone cells contain iodopsin (a general term for cone pigments, though specific photopsins are more accurate).

Why other options are wrong:

(B) is incorrect: Rod cells are more numerous than cone cells (approx. 120 million rods vs. 6-7 million cones). Rods are responsible for peripheral vision, not central vision. Cones are concentrated in the fovea, responsible for central and high-acuity vision.

(C) is incorrect: Rod cells are responsible for dim light vision and do not mediate color vision. Cone cells are responsible for bright light and color vision.

(D) is incorrect: Rod cells are highly sensitive to dim light, while cone cells are less sensitive to dim light but provide high visual acuity.

Common Mistake: Students often confuse which photoreceptor is responsible for color vision versus dim light vision, and their relative numbers or distribution in the retina.

NEET Tip / Mnemonic: Remember "Cones for Color and Central vision" and "Rods for Rhodopsin and dim light (Really dark) vision."

Correct Answer: (A)