A. graphite rods
B. concrete shield
C. iron rods
D. boron rods
Option A – graphite rods
Moderation is the process of the reduction of the initial high speed (high kinetic energy) of the free neutron.
Since energy is conserved, this reduction of the neutron speed takes place by transfer of energy to a material called a moderator.
There are several different types of moderating materials, and each have places where they are used more effectively.
Typically-used moderator materials include heavy water, light water, and graphite.