When connecting an ohmmeter, what must be ensured?

When using an ohmmeter to measure resistance, it is essential to ensure that the power is off. This is because an ohmmeter functions by passing a small current through the resistor being tested and measuring the voltage drop across it to calculate the resistance. If the device being tested is powered on, it can introduce additional voltage and current that may interfere with the measurement, potentially damaging the ohmmeter or the device itself.

By ensuring the power is off, the ohmmeter can accurately measure the resistance without any influence from active circuits. This practice maintains safety for both the user and the instruments involved, allowing for reliable readings without the risk of electrical shocks or erroneous data. In the context of the options provided, this understanding confirms that keeping the power off is the necessary step before connecting the ohmmeter for an accurate resistance measurement.