What purpose do control rods serve in a nuclear power plant?

to absorb neutrons before the neutrons can initiate more fusion reactions
to produce neutrons and initiate more fission reactions
to absorb neutrons before the neutrons can initiate more fission reactions
to absorb neutrons before the neutrons can fuse together

To absorb neutrons before the neutrons can initiate more fusion reactions.

Explanation:

Control rods, usually made of compounds like graphite, absorb neutrons and prevent them from going everywhere and bombarding other molecules of the radioactive compound being used, usually U-238, and prevent a chain reaction from starting. When U-238 is bombarded with a neutron, it releases 3 others, which hit 3 molecules of U-238, releasing 27 neutrons in total, and so on. This must be controlled, otherwise an uncontrolled chain reaction could easily become highly explosive (Nuclear bombs that utilise U-238 use this principle to cause a chain reaction at a larger scale that releases huge amounts of energy that cause the initial explosion when the atom bomb is detonated).