Which of the following can damage a multimeter?

When a multimeter is set to measure resistance (ohms), it uses its own internal battery to send a tiny, calibrated current through the component being tested. The meter's internal circuitry is highly sensitive and configured to measure the resulting voltage drop to calculate resistance. If you accidentally connect the meter probes to a live, energized circuit (a voltage source) while it is still in the resistance setting, external voltage is forced back into the meter. This sudden influx of electrical energy will overload the sensitive measurement circuits, potentially blowing the meter's internal fuse or permanently destroying its components. The other options do not pose a danger to the meter. Attempting to measure resistance on the voltage setting will simply result in a reading of zero, as no current is flowing. Failing to connect a ground probe just prevents a complete circuit, causing the meter to display no reading at all.