Which of the following precautions should be taken when measuring in-circuit resistance with an ohmmeter?

Ohmmeters determine resistance by applying a known, small voltage from an internal battery to the circuit and measuring the resulting current. If the circuit is powered on while you attempt this, the external voltage from the circuit will interfere with the meter's internal voltage. This clash will not only cause wildly inaccurate resistance readings, but it can also blow the meter's internal fuse or severely damage its delicate measurement circuitry. Therefore, you must always disconnect the power source and discharge any large capacitors before testing in-circuit resistance. Distractors like ensuring applied voltages are correct apply to voltage testing, not resistance testing.