When an AC voltage is being measured across a 15 Ù resistor which is carrying a current

of 12 amps, the selector switch should be set to the AC voltage position of

V = IR = 12 * 15 = 180 Volts.

Set switch to 200 volt Range or higher.

To determine the appropriate position to set the selector switch to measure AC voltage across a resistor, you will need to consider the voltage and current values given.

In this case, the resistor has a resistance value of 15 Ω and is carrying a current of 12 amps.

Since we are measuring AC voltage, we need to use the appropriate AC voltage measurement setting on the selector switch. The AC voltage measurement setting is typically denoted by a V with a wavy line (~) symbol next to it or by a specific label such as "AC V" or "V~".

To select the correct range on the AC voltage setting, we need to consider the expected voltage across the resistor. If you have an idea of the approximate voltage value, you can start with the closest range on the selector switch. It is better to start with a higher range to avoid overloading the instrument.

For example, if you expect the voltage across the 15 Ω resistor to be around 120 volts, you would select a range that can measure that voltage, such as 200V or 500V AC.

If you are unsure of the voltage range, you can start with the highest range available on the AC voltage setting and then gradually decrease it until you get a suitable reading on the measurement display. Adjust the range until you are able to obtain the most precise measurement.

Remember to set your multimeter to the AC voltage measurement setting on the selector switch and make sure to follow the safety precautions provided by the manufacturer.