Imagine a question and stream-of-consciousness explanation for which this is the answer: hydrogen isotopes such as deuterium and tritium
Question: Fusion processes require fuel and a highly confined environment with a high temperature and pressure, to create a plasma in which fusion can occur. In stars, the most common fuel is hydrogen, and gravity creates the high temperature and confinement needed for fusion. Fusion reactors generally use hydrogen isotopes such as deuterium and tritium, which react more easily, and create a confined plasma of millions of degrees using inertial methods (laser) or magnetic methods (tokamak and similar), although many other concepts have been attempted. The major challenges in realising fusion power are to engineer a system that can confine the plasma long enough at high enough temperature and density, for a long term reaction to occur, and for the most common reactions, managing neutrons that are released during the reaction, which over time can degrade many common materials used within the reaction chamber.

What is the primary fuel that would be used in fusion reactors?
Stream-of-consciousness: The relevant information to answer the above question is: Fusion reactors generally use hydrogen isotopes such as deuterium and tritium, which react more easily, and create a confined plasma of millions of degrees using inertial methods (laser) or magnetic methods (tokamak and similar), although many other concepts have been attempted.