The most practical fuel for a thermonuclear reactor, both from economical and nuclear consideration is

Answer: Option C

Most commercial nuclear reactors use normal water (also called light water) as a neutron moderator.
Some reactor designs, such as the CANDU reactor, use heavy water.
A heavy water molecule contains deuterium (Hydrogen-2), which is an isotope of hydrogen that has a neutron in its nucleus in addition to the proton.