Which letter on the graph above represents fusion?

The figure here is missing, so we cannot say what is the corresponding letter; however we can describe what nuclear fusion is.

Nuclear fusion is a nuclear process in which light nuclei fuse together into a heavier nucleus. In the reaction, the total mass of the initial nuclei is less than the total mass of the final nucleus, so part of the mass has been converted into energy, according to the equation:

where

is the energy released

c is the speed of light

is the mass defect, which is the mass difference between reactants and products.

Since the factor is very large, we can see that the amount of energy released by a process of nuclear fusion is huge.

An example of nuclear fusion is the fusion of hydrogen nuclei into  helium. This process occurs in the core of the stars, and it is the main process through which stars produce energy. In order to occur, the temperature inside the core must be at the order of several million degrees, because the nuclei must overcome the electrostatic repulsion between them, so that they can come close enough to fuse together: for this reason, nuclear fusion is difficult to achieve on Earth, since it is very challenging to recreate the same conditions of temperature and pressure that we have in the core of the stars.