Which optical instrument is used to observe distant objects?

A telescope is an optical instrument designed specifically to observe distant objects in space. It works by collecting and focusing light from celestial bodies such as stars, planets, and galaxies. Telescopes can be either refracting (using lenses) or reflecting (using mirrors) in design, and they come in various sizes and configurations to suit different observational needs. Telescopes are crucial tools in astronomy, allowing astronomers to explore the universe and study celestial phenomena. Option A, a simple microscope, is used for viewing small objects at low magnification and is not suitable for observing distant celestial objects. Option B, a compound microscope, is used for magnifying microscopic specimens in biological and medical research. Option C, an electron microscope, is used to observe very small objects at high magnification using a beam of electrons rather than light. Therefore, the correct optical instrument for observing distant objects, such as those in space, is the telescope (option D).