To test the state of charge of a nickel-cadmium battery, what should be used?

Correct Answer: Option C — voltmeter

Nickel-cadmium batteries maintain a relatively constant specific gravity throughout their discharge cycle, rendering hydrometers ineffective for state-of-charge testing. Consequently, a voltmeter is the standard tool used to measure terminal voltage, which provides a reliable indication of the battery's charge level under specific load conditions.