Question

the primary mirror in a reflecting telescope is a plane mirror. concave mirror. convex mirror.

Explanation:

A reflecting telescope uses a concave mirror as the primary mirror. A concave mirror can gather and focus light from distant objects. A plane mirror would just reflect light without focusing it in a useful way for magnification in a telescope. A convex mirror diverges light, which is not suitable for collecting light from distant celestial objects to form an image in a telescope.

Answer:

concave mirror.