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

When working with high voltages, physical safety is the top priority. Voltmeters and their test leads are manufactured with specific insulation ratings designed to protect the user from electrical shock. If you measure a voltage that exceeds the rating of your equipment, the insulation can break down, causing a dangerous arc or electrocution. Voltmeters actually require a high impedance, not low, so they do not draw excessive current and alter the circuit being measured. Grounding a circuit through the voltmeter is incorrect and dangerous, as it would create a short circuit, and frequency settings do not address the primary safety hazard of high voltage insulation breakdown.