The control rod in a nuclear reactor is made of (A) uranium (B) cadmium (C) plutonium (D) graphite

Control rods are used in nuclear reactors to control the rate of nuclear fission. They do this by absorbing neutrons, which prevents them from causing further fissions. As a result, control rods play a critical role in regulating the reaction rate and ensuring the safety of the reactor.

Now, we will look at each material and determine whether it is suitable for use as a control rod. (A) Uranium: Uranium is a key fuel for nuclear reactors but not an ideal material for control rods. Control rods need to have a high neutron-absorbing capacity. Uranium can undergo fission when it absorbs a neutron, contributing to the chain reaction rather than controlling it. Therefore, uranium is not a suitable material for control rods. (B) Cadmium: Cadmium is a highly effective neutron absorber, making it an ideal material for control rods. It has a high neutron-absorbing capacity and doesn't contribute to the fission process, making it an effective material for controlling the rate of the nuclear reaction. (C) Plutonium: Like uranium, plutonium can be used as a fuel for nuclear reactors. However, it is not suitable for use as a control rod, as it also participates in the fission process when it absorbs neutrons. This means that it wouldn't effectively control the rate of the nuclear reaction. (D) Graphite: Graphite is used in nuclear reactors as a moderator, meaning it is not suitable for use as a control rod. Moderators slow down neutrons but don't absorb them, making them less effective in controlling the rate of nuclear fission.

After evaluating each material, it is clear that cadmium (B) is the most suitable material for a control rod in a nuclear reactor. It has a high neutron-absorbing capacity, doesn't contribute to the fission process, and effectively controls the rate of the nuclear reaction. Therefore, the correct answer is (B) cadmium.