What is an electronvolt (eV) used to measure?

An electronvolt (eV) is defined as the amount of energy gained or lost by an electron when it is moved through an electric potential difference of one volt. This unit of energy is particularly useful in fields like atomic and particle physics. When an electron is subjected to a potential difference of one volt, it experiences a force and, as a result, gains energy equal to one electronvolt. This makes it a practical measure for analyzing interactions at the atomic and subatomic levels, including ionization energy, kinetic energy of particles, and photon energies in various systems.

While the charge of an electron, the mass of a proton, and other energy measures have their own respective units or contexts, the specific relationship outlined in option B encapsulates the fundamental definition of an electronvolt, making it the correct answer. Understanding the electronvolt is crucial when interpreting data related to electron behavior in electric fields and various physical phenomena.