Which of the following is a characteristic of a conductor?

A conductor is characterized by its ability to easily allow electric charge to flow through it. This property is due to the presence of free electrons in the material, which can move freely and carry electrical current when a voltage is applied. Materials such as copper and aluminum are common conductors because they have a structure that facilitates the movement of these electrons.

In contrast, options that describe restricting electric charge flow, functioning as an insulator, or having high resistance do not align with the fundamental properties of conductors. Insulators are materials that prevent or severely limit the flow of electric current, typically having very few free electrons. High resistance also indicates a material's inability to conduct electricity efficiently, which again is not a characteristic of conductors. Therefore, the defining feature of a conductor is its capacity to facilitate easy flow of electric charges, making the correct answer clear.