Which instrument is used for measuring resistance on a direct ground stud?

The correct choice is the ohmmeter because it is specifically designed to measure electrical resistance. When measuring the resistance of a direct ground stud, understanding the resistance value is crucial to ensure that it provides an effective ground connection. An ohmmeter applies a small voltage through the circuit and measures the current flowing, allowing it to calculate and display the resistance in ohms.

While voltmeters measure voltage and ammeters measure current, they do not provide readings of resistance directly. A multimeter could measure resistance if it has that function, but the instrument's primary purpose is broader, encompassing the measurement of voltage and current as well. Since the question specifically asks for a device tailored for measuring resistance, the ohmmeter stands out as the most appropriate and specialized tool for this task.