Which of the following precautions should be taken when measuring in-circuit resistance with an ohmmeter?
Why An ohmmeter works by applying its own small internal voltage and measuring the resulting current, then computing R = V/I. Any external voltage in the circuit adds to or fights that internal source, giving meaningless readings and possibly damaging the meter. So always remove power, and discharge any large capacitors, before touching an ohmmeter to a circuit.
Watch out The choices about correct applied voltage or correct operating frequency describe checks you would make on a live circuit with a voltmeter or frequency counter, which is exactly the opposite of what resistance measurement requires.
Ohmmeter brings its own battery: power off, caps discharged, then measure.