Question

two isotopes commonly used in fusion research are which of the following, along with tritium?
protium
deuterium
helium
lithium

Explanation:

In nuclear fusion research, the most common fuel combination is deuterium (an isotope of hydrogen) and tritium. Protium is the most abundant hydrogen isotope but not used in typical fusion research due to its high fusion energy requirement. Helium is a product of deuterium-tritium fusion, not a fuel. Lithium is used to breed tritium but is not a primary fusion fuel alongside tritium.

Answer:

Deuterium