Which instrument would you use to measure electric potential?

In electronics and physics, "electric potential" or "potential difference" is the formal scientific term for what we commonly call voltage. Since voltage is measured in units called volts, the correct instrument to measure electric potential is a voltmeter. The other options serve completely different purposes in a circuit. An ammeter measures electric current (in amperes), and an ohmmeter measures electrical resistance (in ohms). A potentiometer is not a primary measurement device at all; it is a variable resistor often used as a physical control dial, like a volume knob.