Final answer:
The core of the Sun is extremely hot primarily due to nuclear reactions that fuse hydrogen nuclei into helium, releasing a tremendous amount of energy. This process occurs in conditions of extreme heat and pressure, with temperatures around 15 million K, confirmed through theoretical models and observations.
Step-by-step explanation:
The core of the Sun is extremely hot due to nuclear reactions that occur within it. In the Sun's core, the density and temperature are so high that hydrogen nuclei (1H) can undergo fusion to form helium, releasing immense amounts of energy. This process requires temperatures exceeding 12 million K, and the Sun's core is actually around 15 million K, which has been confirmed through complex mathematical models and observations. Additionally, the extreme pressure in the core contributes to the conditions necessary for nuclear fusion. This nuclear fusion in the Sun's core is the source of its energy, rather than any form of chemical reactions or magnetic fields, making option A the correct answer. The core is roughly 20% the size of the Sun but contains about one-third of its mass, reiterating how extreme the conditions are in this region.