How is electrical current defined?

The definition of electrical current is centered around the flow of electrons, which is precisely captured in the correct choice. Current is fundamentally the movement of charge carriers, and in most conductive materials, particularly metals, these charge carriers are electrons. When a potential difference (voltage) is applied across a conductor, it creates an electric field that prompts the electrons to move from the negative terminal of the power source towards the positive terminal. This movement of electrons constitutes an electric current, measured in amperes (A).

In contrast to the incorrect options, the flow of voltage through a circuit does not define current itself but rather describes a condition needed to drive the current. Electrical resistance refers to the opposition to current flow but does not define what current is. Lastly, the idea of protons flowing through a conductor is fundamentally inaccurate, as protons are bound within atomic nuclei and do not move through conductive materials in the same way electrons do. Thus, the flow of electrons from a source to a lead gives a clear and accurate definition of electrical current.