Question

structure in the eye that focuses light rays on the retina
mirrors
viewfinders
sensors
filters
lens

Explanation:

In the eye, the lens is the structure that focuses light rays onto the retina. Mirrors are not part of the eye's structure for this function. Viewfinders are related to cameras, not eyes. Sensors in general terms are too broad and not specific to this eye - focusing role. Filters also do not have the function of focusing light onto the retina in the eye.

Answer:

lens