What does the Ohmmeter require for its operation?

An ohmmeter is specifically designed to measure resistance in electrical circuits and components. For its operation, it has its own internal electrical power source, typically a battery. This built-in power allows the ohmmeter to send a small current through the component being tested, enabling it to calculate resistance based on the voltage drop across that component.

The option indicating that it requires an external power source would imply the need for an additional supply, which is not the case for a typical ohmmeter. Similarly, while some devices may incorporate battery backup, that isn't a requirement for ohmmeters as they are designed to operate off their internal battery alone. The last option, needing an AC power source, does not apply to most handheld or portable ohmmeters, which operate independently on DC power provided by their internal batteries. Thus, having its own electrical power is the defining characteristic that allows an ohmmeter to function properly.