The correct answer to this question is A, Hydrogen. Fusion is a nuclear process. Energy is made during this process, but the smashing together of atoms. Fusion uses stars, such as the sun, to produce heat and light. On Earth, fusion occurs by combining tritium and deuterium, which are two isotopes of hydrogen.
Hydrogen is an element and is the lightest of them all. It is made of just one proton and one electron. Two nuclei must be brought close together to activate for fusion to occur. With this activation, fusion occurs typically at very high temperatures and a high density.