Right option is (c) thermo-motive
Easy explanation: When two wires of different material are joined together at either ends, forming two junctions which are maintained at a different temperature, a thermo-motive force is generated causing a current to flow around the circuit. This arrangement is called a thermocouple. The junction at a lower temperature in the thermocouple is called the reference temperature. The junction at a higher temperature in thermocouple is termed as measuring junction.
