If I am not wrong, I believe the answer is Producers. The reason as to why I say that producers is the answer is because in every food chain, there needs to be a base. That base is what provides the starting energy that will later on move up in the food chain.
There is only one main element involved in the fusion reactions inside the sun; Hydrogen, but if you need a second element then it is more than likely that the question is asking for the isotope of hydrogen that is called <span>deuterium.
So basically, Hydrogen and Deuterium; an isotope of hydrogen</span>
