A

The thermocouple operates based on the Seebeck effect.
Step 1: Understand the Seebeck effect. This phenomenon occurs when two different conductors are joined at two junctions, and a temperature difference is maintained between these junctions, producing a voltage.
Step 2: For thermocouples, when one junction is heated and the other is maintained at a lower temperature, an electromotive force (emf) is generated as a result of the different thermoelectric properties of the materials.
Step 3: The other options do relate to thermoelectric effects but serve different functions:
- The Thomson effect describes the heating or cooling of a current-carrying conductor in a temperature gradient.
- The Peltier effect refers to heating or cooling at the junction of two different materials when a current flows through them.
- The Joule effect describes the heating of a conductor due to electric current.
Therefore, the correct answer is Seebeck effect (Option A).