Which eye type is made up of only a single lens?

The correct choice is the simple eye, which is characterized by having only a single lens. This type of eye is exemplified by the eyes found in certain animals, such as vertebrates. Simple eyes can provide a focused image on the retina and are often capable of some degree of image processing.

In contrast, complex eyes consist of multiple lenses and may include other specialized structures that allow for more intricate vision capabilities, such as depth perception and color discrimination. Compound eyes, typical in many arthropods, consist of numerous small lenses (ommatidia) that collectively offer a wide field of view but with lower resolution. Multifocal eyes involve the presence of different focal points for viewing, incorporating multiple lenses with varying refractive properties, but they do not represent the simple lens structure of a simple eye.