Question

In a nuclear reactor, the control rods are:
a. made of graphite
b. used to keep the reactor cool
c. used to absorb free neutrons
d. all of the above
e. none of the above

Answer 1

all are the right but e is not sufficient

Answer 2

the answer is none of the above.
because as per me control rods are made from cadmium and they are used only in critical state when there are extra neutrons emitted in the reactor core and to stabilise the reactor.

Cadmium rods are used.

and for cooling the reactor continuous supply of water is given through the outer Wall.

hope my answer cleared and helped to satisfy your requirements.