The 'coolants' which can be used in the reactor of a nuclear power station are:

1. liquid mercury and nitrogen dioxide
2. liquid sodium and carbon dioxide
3. liquid ammonia and carbon monoxide
4. liquid boron and uranium oxide

(b) liquid sodium and carbon dioxide

Explanation

Liquid sodium and carbon dioxide act as 'coolants', therefore they are used in order to transfer the heat produced in the reactor by fission to the heat exchanger, for converting water into steam.