What is the primary purpose of a voltmeter in electrical practice?

A voltmeter is specifically designed to measure the voltage, or potential difference, across two points in an electrical circuit. This function is essential for understanding how much electrical energy is available to push current through a circuit, which can help in diagnosing and analyzing electrical systems. By connecting a voltmeter in parallel with a component, one can obtain an accurate reading of how much voltage is being dropped across that component, which is critical for determining if the component is functioning correctly or if there's an issue in the circuit.

Measuring current or resistance, calculating power consumption, and the other options involve different types of measurements that utilize different devices such as ammeters for current, ohmmeters for resistance, or wattmeters for power. Those instruments have distinct functionalities that do not overlap with the voltmeter's primary purpose of measuring voltage. Thus, option C stands out as the correct choice, as it clearly aligns with the voltmeter's role in electrical practice.