Final answer:
Three hydrogen atoms need to fuse in order to form one helium atom through nuclear fusion.
Step-by-step explanation:
The fusion reaction that forms one helium atom from four hydrogen atoms is a two-step process. Firstly, at high temperatures, two hydrogen protons combine to form a deuterium nucleus, containing one proton and one neutron. This step requires one hydrogen atom to fuse. Then, another hydrogen proton combines with the deuterium nucleus to form a helium nucleus with two protons and one neutron. This step requires a total of three hydrogen atoms to fuse. Therefore, the correct answer is option C) 3.