Which of these parts of a reflecting telescope is used to gather and focus light?

a. lens

b. tube

c. convex mirror

d. concave mirror

Option (B), (C) and (D)

Explanation: In a reflecting telescope, concave and convex mirrors are used to focus light, in a tube.

In case of a reflecting telescope, a concave mirror is usually placed at one end of the telescope's tube whereas the other end is kept open from where the light is allowed to pass through. In between this two end of the tube, a comparatively small flat mirror is used, placing it at an angle of approximately 45°, that further reflects the reflected light to the eyepiece containing a convex mirror where a large image is observed.

Thus the above mentioned options (B), (C) and (D) are used in a reflecting telescope.

a corroding iron, if that's one of your choices