Final answer:
Reflecting telescopes use a concave primary mirror to focus light, avoiding chromatic aberration and allowing for easier manufacturing compared to lens-based refractors. The correct option is A.
Step-by-step explanation:
Telescopes with a concave mirror in the aperture are known as reflecting telescopes. Reflecting telescopes use a concave primary mirror to collect light and bring it to a focus. This design solves problems such as chromatic aberration that are inherent in refracting telescopes, which use lenses.
The first successful reflecting telescope was built by Isaac Newton in 1668. Most modern large astronomical telescopes are reflectors, as they are easier to manufacture and support compared to lens-based refractors, and because the light does not need to pass through glass, which must be of high quality and free from imperfections.
Moreover, the reflective surface is coated with materials like silver, aluminum, or gold to maximise reflectivity. Option A. is the correct one.