In a nuclear reactor :

1. moderator is used to control the number of neutrons.
2. controlling rods are used to slow down the neutrons.
3. moderator is used to slow down the neutrons.
4. coolant is used to slow down the neutrons.

moderator is used to slow down the neutrons.

Reason — In a nuclear reactor, different components perform specific roles.

• The moderator (such as graphite or heavy water) is used to slow down fast-moving neutrons so that they can effectively cause further fission reactions.
• Control rods (made of materials like cadmium) are used to absorb excess neutrons and regulate the chain reaction, not to slow them down.
• The coolant is used to remove heat from the reactor core, not to affect neutron speed.

Therefore, correct statement is : the moderator is used to slow down the neutrons.