The answer is "a. Refracting telescopes".
Refracting Telescopes ordinarily known as refractors, telescopes of this kind are utilized to look at the noticeable light area of the electromagnetic range. Run of the mill utilizes incorporate survey the Moon, different objects of the close planetary system, for example, Jupiter and Mars, and twofold stars. The name refractor is gotten from the term refraction, which is the bowing of light when it goes starting with one medium then onto the next of various thickness - e.g., from air to glass. The glass is alluded to as a focal point and may have at least one parts.