Step 1: Understand the quantity.
Electric potential difference, or voltage, is the work done to move a unit positive charge between two points.
Step 2: Identify the unit.
Its SI unit is the volt, named after Alessandro Volta.
Step 3: Definition.
One volt is defined as $1\,V = \dfrac{1\,J}{1\,C}$, that is one joule per coulomb.
Step 4: Rule out others.
The ampere measures current, the watt measures power, and the ohm measures resistance.
Step 5: Conclude.
The SI unit of voltage is the volt.
Answer: Volt