What sort of device is commonly used to separate the colors of light from white light?

a prism?

a lens?

a telescope?

or a rainbow ... ?

I would say a prism because it's the "fine line" between clear and a rainbow

A prism

Explanation:

1. A prism is a transparent optical device with polished surfaces that refracts light. This property of a prism causes various colors of light to disperse or refract according to their wavelengths, at different angles as white light crosses into the prism (from a less dense medium into a denser medium, air into glass), this is similar to the naturally occurring rainbow phenomenon.

Hence, since all colors have different wavelengths, a prism is the device used to separate white light into all colors of the visible spectrum.

2. A lens is an optical device that is used to disperse or converge visible light.

There are two types of lens, simple and complex lens, however neither of both types is used for the purpose of separation of colors of light.

3. A telescope is an instrument that uses one or multiple lenses to focus light rays. This also does not separate colors of light.

4. A rainbow is not a device, a rainbow is a naturally occurring phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky.