In Cassegrain telescope, the objective is a:

The correct answer is Option (2) → Large concave mirror

A Cassegrain telescope is a reflecting telescope. Its objective is the primary mirror, a large concave (parabolic) mirror with a hole at its centre that collects the light from the distant object. A small convex secondary mirror reflects the converging light back through the hole to the eyepiece.

Because a large aperture gathers more light and gives better resolving power, the objective is made large; it is a mirror, not a lens, so Option (4) is excluded.

NCERT (2026-27):
Telescope – “… For these reasons, modern telescopes use a concave mirror rather than a lens for the objective. Telescopes with mirror objectives are called reflecting telescopes.”

Telescope – “… One such arrangement using a convex secondary mirror to focus the incident light, which now passes through a hole in the objective primary mirror, is shown in Fig. 9.26. This is known as a Cassegrain telescope, after its inventor.”