Final answer:
The third step in the fusion process is two helium-3 nuclei from the earlier step combining, leading to the release of two protons that can initiate the fusion cycle anew. This step is crucial to continue the fusion process, which is dependent on high temperatures and fuel density. The correct option is b.
Step-by-step explanation:
The third step in the fusion process, specifically when referring to the fusion of hydrogen into helium as occurs in the Sun, involves two helium-3 nuclei combining. This process, detailed in the Proton-Proton Chain Step 3, comes after the second step in which these helium-3 nuclei are formed. The fusion process under discussion requires several conditions, such as extremely high temperatures above 15,000,000 K to form plasma, increased fuel density, and adequate confinement time to yield significant energy through the fusion reaction.
This fusion stage is crucial for continuing the chain reaction, as the two protons released after this combination possess the energy required to collide with other protons, thus, restarting the fusion cycle from step one again. Therefore, for the specific question asked, the third step in the fusion process is forming a plasma.