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.