Which piece of test equipment must be connected in parallel to measure voltage?

To measure voltage in a circuit, the appropriate piece of test equipment is a voltmeter, which must be connected in parallel to the component across which the voltage needs to be measured. This parallel connection allows the voltmeter to measure the potential difference without significantly altering the current flowing through the circuit.

When connected in parallel, the voltmeter can provide an accurate reading of the voltage because it shares the same voltage potential as the components in that branch of the circuit. The design of a voltmeter has high internal resistance, minimizing its impact on the overall circuit operation and ensuring that it does not draw significant current.

In contrast, other devices such as an ammeter measure current by being connected in series with the circuit, drawing current through themselves, which would not allow for voltage measurement. An ohm meter, used to measure resistance, is also typically connected in a manner that does not record voltage directly. A frequency meter is primarily used to measure the frequency of AC signals and does not measure voltage directly either. Therefore, the requirement for measuring voltage accurately and effectively is fulfilled by using a voltmeter in parallel, making it the correct answer.