What is used as a moderator in a nuclear reactor?

Nuclear fission is best explained by
[$AIPMT$ $2000$]

In a nuclear reactor, the main purpose of the moderator is to

The binding energy per nucleon for ${}_1^2H$ and ${}_2^4He$ are $1.1 \; MeV$ and $7.1 \; MeV$ respectively. The energy released in $MeV$ when two ${}_1^2H$ nuclei fuse to form one ${}_2^4He$ nucleus is:

By which process is the energy released from the Sun?

Fusion reaction is initiated with the help of