Which of the following precautions should be taken when measuring high voltages with a voltmeter?

When measuring high voltages, your physical safety relies entirely on the insulation provided by your testing equipment. You must ensure that both the voltmeter and its test leads are explicitly rated to handle voltages higher than what you intend to measure. If the voltage exceeds the design rating of the meter or the insulation on the leads, the electrical current can arc through the plastic directly into your hand, resulting in a severe or lethal shock. Using appropriately rated CAT (Category) equipment guarantees the insulation is robust enough for the job. The distractors suggest dangerous or nonsensical actions. Using a voltmeter with very low impedance on a high-voltage circuit would draw massive current and cause a catastrophic short circuit. Grounding the circuit through the voltmeter would similarly cause a dead short. Finally, voltage amplitude, not frequency, dictates the required insulation rating.