What is the purpose of an ammeter?

An ammeter is specifically designed to measure the electric current flowing through a circuit. It works by being connected in series with the circuit to ensure that all the current flowing through that part of the circuit passes through the ammeter. The unit of measurement for electric current is amperes, and an ammeter displays this value.

In contrast, measuring voltage requires a voltmeter, which is connected in parallel across a component or section of the circuit. Resistance is measured with an ohmmeter, which is not used during normal circuit operation, and frequency measurements are typically made using an oscilloscope or frequency counter. Therefore, the primary and defining function of an ammeter is its ability to accurately gauge current, making option B the correct choice.