Detailed Explanation:

The Thomson effect describes the generation of electromotive force (EMF) in a single conductor due to a temperature gradient along its length.

• Core Distinction: Unlike the Seebeck effect (requiring two dissimilar metals), Thomson effect occurs in a single homogeneous conductor with varying temperature.
• Mechanism: Electrons diffuse from hot to cold regions, creating a potential difference proportional to the temperature gradient.
• Therefore, potential variation due to temperature variation in a single conductor is the Thomson effect, making Option 4 the correct answer.