Final answer:
Voltage is measured between two points because it is a relative measure that requires a reference. It is a scalar quantity, and the zero voltage point is arbitrary, similar to the zero for gravitational potential energy.
Step-by-step explanation:
Voltages are always measured between two points because voltage is a relative measure, requiring two points for reference. The correct answer is d) Voltage is a relative measure, requiring two points for reference. Voltage, also known as electric potential difference, is understood to be the difference in potential between two points. For instance, a battery has two terminals, and the voltage of the battery is the potential difference between these terminals.
Furthermore, the electric potential is a scalar quantity, meaning it has magnitude but no direction, unlike the electric field, which is a vector quantity that has both magnitude and direction. The arbitrary nature of a zero voltage point is similar to choosing an arbitrary zero for gravitational potential energy.