The substance most likely to undergo nuclear fusion is hydrogen, specifically its isotope deuterium.

Here's why:

* Low Coulomb Barrier: Deuterium has only one proton, making its nucleus smaller and less positively charged. This results in a lower Coulomb barrier, the electrostatic repulsion between the positively charged nuclei that must be overcome for fusion to occur.

* High Binding Energy: The fusion of deuterium nuclei releases a large amount of energy, as the resulting helium nucleus is more tightly bound than the original deuterium nuclei.

* Abundance: Deuterium is a relatively abundant isotope of hydrogen, making it a readily available fuel source for fusion.

While other light elements like tritium and helium-3 can also undergo fusion, they are less abundant and have higher Coulomb barriers, making them less likely to fuse under typical conditions.