Option (C) ciliary muscles is correct. According to the NCERT textbook section 10.1.1 on Power of Accommodation, the eye lens is composed of a fibrous, jelly-like material whose curvature can be modified by the ciliary muscles. The text explicitly states that 'The change in the curvature of the eye lens can thus change its focal length.' When the ciliary muscles are relaxed, the lens becomes thin and its focal length increases for seeing distant objects. When the ciliary muscles contract, the lens becomes thicker and its focal length decreases for seeing nearby objects. The other options—pupil, retina, and iris—are not responsible for changing the focal length of the eye lens.