Question

which of the following structures focuses incoming light rays onto the retina?
a iris
b lens
c pupil
d ciliary body

Explanation:

The lens is responsible for focusing incoming light rays onto the retina. The iris controls the size of the pupil (which is just an opening, not a focusing structure). The ciliary body is involved in accommodation (changing the shape of the lens) but the main structure for focusing light onto the retina is the lens.

Answer:

B. lens