a. H₂ 

b. D₂ 

c. H₂O 

d. D₂O 

e. Beryllium 

f. Graphite.

The functions of a good moderator are .

a. It has to thermalise neutrons effectively. 

b. There should be only a minimum absorption of neutrons. 

c. It should not be toxic. 

d. It should not be inflammable.

 In case of Hydrogen (H₂) though it is a effective sheatherer it cannot be used as a moderator because it is a gas and there is a lot of distance between atoms. A neutron cannot meet the nucleus in a definite manner.

For D₂ also the same problem as this is also gas and it cannot be used as a moderator. In case of H₂O it is a good scatterer but is absorbs neutrons. Its moderation ratio is 72. So it cannot be used as a moderator.

In case of D₂O though it is not as effective scatterer as that of H₂O it has minimum absorption of neutrons. It has a moderating ratio of 21000. This is an ideal moderator.

In case of Beryllium it is a toxic material. Therefore cannot be used. Graphite absorbs neutrons and is inflammable and therefore cannot be used. So from the above statements the D₂O is the good moderator material.