Final answer:
The Sun's energy and composition are provided by the nuclear fusion of hydrogen into helium, which occurs in the core of the Sun and releases a significant amount of energy.
Step-by-step explanation:
The energy and composition of the Sun are provided by the nuclear fusion of hydrogen into helium. This process, which occurs in the Sun's core, involves the fusion of four protons (hydrogen nuclei) into one alpha particle (helium nucleus), accompanied by the release of two positrons, two neutrinos, and a significant amount of energy due to the mass difference between the reactants and the products.
Hence, the correct answer to the question is D. the fusion of hydrogen into helium. The Sun's energy generation process is crucial to understanding both solar and stellar dynamics. It is also an example of converting mass into energy, as described by Einstein's famous equation E = mc².