What does the mass number represent?

The mass number is defined as the total number of protons and neutrons present in an atomic nucleus. Protons contribute a single unit to the mass number, as do neutrons, while electrons, having negligible mass, are not included in this count. This characteristic of the mass number makes choice C the right answer.

The relevance of understanding mass number lies in its role in identifying isotopes of an element, which differ in neutron number but belong to the same element due to having the same number of protons. This knowledge is fundamental in nuclear chemistry and its applications, including radiation safety, as it influences the stability of the nucleus and the type of radiation emitted during decay.

The other choices do not convey the full definition of mass number: focusing only on protons would not account for the presence of neutrons that significantly influence the atomic mass and nuclear stability. Similarly, identifying mass number solely with neutrons neglects the contribution of protons, while describing it as a weighted average of isotopic masses misrepresents what mass number is. The mass number is a discrete value rather than an average, making C the accurate and complete representation.