What must be done before making resistance measurements?

Before making resistance measurements, it is essential to ensure that no voltages are present in the circuit. This is crucial because applying a multimeter to measure resistance while there is voltage can damage the meter and give inaccurate readings. Voltage can cause current to flow through the multimeter, leading to potential harm to the internal components of the meter.

In addition, voltage in the circuit can lead to erroneous resistance readings since ohmmeters operate by passing a small current through the circuit, and any existing voltage will interfere with this operation. Ensuring that the circuit is completely de-energized, either by turning off the power or verifying that there are no residual voltages, prepares the meter for accurate and safe measurements. This practice is part of standard safety and operational protocols in electronics work.

Other choices, while they may sound reasonable, do not specifically address the pressing need to eliminate voltage presence in the circuit prior to resistance measurement.