(d) Assertion (A) is false, but Reason (R) is true.
When ciliary muscles contract, the eye lens becomes thicker (not thin), decreasing focal length to focus nearby objects. However, Reason (R) is correct — ciliary muscles do control the power (focal length) of the eye lens.
Source: Chapter 10, Section 10.1.1 Power of Accommodation
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The passage clearly states: "When you are looking at objects closer to the eye, the ciliary muscles contract. This increases the curvature of the eye lens. The eye lens then becomes thicker." So the Assertion is false — contraction makes the lens thick, not thin (the lens becomes thin when muscles are relaxed). The Reason is true since ciliary muscles change the curvature and hence the focal length (power) of the lens. This makes option (d) correct.