How does an ammeter differ from a voltmeter in operation?

An ammeter is designed specifically to measure the electric current flowing in a circuit, typically displayed in amperes (A). The core function of an ammeter is to be integrated into the circuit through which the current flows, allowing it to provide a direct measurement of how much electric charge is moving through that circuit per unit of time. This distinguishes it clearly from a voltmeter, which measures the electrical potential difference (voltage) between two points in the circuit.

In contrast, the other options address different functionalities that do not align with what an ammeter does. While an ammeter focuses solely on current, other instruments are used for measuring AC current, power (in watts), or resistance, which are the domains of different types of meters. Hence, option C is the accurate representation of an ammeter's operation compared to a voltmeter.