Final answer:
The Sun is mostly made up of hydrogen (73% by mass, 92% by number of atoms) and helium (25% by mass). The rest of the Sun (2% by mass) is composed of heavier elements. Option b is the best representation of the Sun's composition by number of atoms.
Step-by-step explanation:
Composition of the Sun
The Sun's composition by mass is approximately 73% hydrogen and 25% helium, with all other elements comprising the remaining 2%. The proportion by mass does not directly translate to the proportion by number of atoms due to the different atomic weights of hydrogen and helium. Hydrogen, being the lightest element, makes up about 92% of the total number of atoms in the Sun, which is still consistent with it making up 73% of the Sun's mass. Helium, being heavier, contributes to a smaller percentage of the atoms but a significant portion of the mass.
In comparison with Earth's crust, which has oxygen, silicon, and aluminum as the most abundant elements, this composition starkly varies as the Sun predominantly consists of hydrogen and helium. This elemental makeup is not unique to the Sun as it reflects a common composition among stars. Stellar spectra provide a good determination of star composition, revealing that most stars are composed mostly of hydrogen and helium, while the abundance of heavier elements can differ.
Therefore, the correct answer to the student's question is option b. 90% hydrogen, 9% helium, and 1% heavier elements, which best approximates the actual composition of the Sun by number of atoms, taking into consideration the simplification that the Sun is made up almost entirely of hydrogen and helium.