Which can’t act as moderator in nuclear power plant? 

The correct answer is option 3):(Friona )

Concept:

• The Controlled nuclear fission reaction takes place in a reactor at the nuclear power plant
• The energy produced by the nuclear reactor is used to produce electricity
• Moderator is used to slowing down the neutrons produced during the fission reaction so that the chain reaction can be sustained. Heavy Water is an excellent moderator due to its high moderating ratio and low absorption cross section for neutrons
• Heavy water, graphite, deuterium, paraffin, etc. can act as moderators. These moderators are rich in protons. When fast-moving neutrons collide head-on with the protons of moderator substances, their energies are interchanged and thus the neutrons are slowed down. Such neutrons are called thermal neutrons which cause the fission of U²³⁵ in the fuel. 
• The graphite bricks act as a moderator. They reduce the speed of neutrons and allow a nuclear reaction to be sustained.
• Hence option 3 is wrong