What does a circuit's resistance measure?

A circuit's resistance measures the opposition to the flow of electric current. Resistance is a fundamental property of materials that dictates how easily electric current can pass through them. It is quantified in ohms (Ω) and is crucial in determining how much current will flow for a given voltage applied across the circuit, according to Ohm's Law (V = I × R). When resistance is high, it makes it harder for the current to flow, while a low resistance allows for easier flow.

The other options relate to different electrical concepts. The total voltage in the circuit is a measure of the electric potential difference and does not directly correlate to how much opposition there is to current flow. The overall power output is calculated using current and voltage but does not indicate resistance specifically. Speed of current flow is not a commonly used term in electrical theory; rather, current is measured in amperes and describes the flow of electric charge, but it does not represent the concept of resistance. Therefore, understanding that resistance refers to the opposition to electric current is essential in the study of electrical circuits.