A

Step 1: Understand the components of a reflecting telescope. A reflecting telescope typically uses mirrors to collect and focus light, as opposed to lenses which are used in refracting telescopes.
Step 2: Identify the type of mirror used in reflecting telescopes. Reflecting telescopes primarily utilize a concave mirror, which is curved inward and can focus parallel rays of light to a focal point, allowing for the creation of clear, magnified images of distant celestial objects.
Step 3: Eliminate the other options:
- Option B: A convex mirror diverges light rays and is not suitable for focusing light.
- Option C: A prism is used in some optical instruments but not in a standard reflecting telescope.
- Option D: A plano-convex lens is characteristic of refracting telescopes.
Conclusion: Therefore, the correct answer is Option A, as a concave mirror is indeed utilized in reflecting telescopes.