Which of the following is a source of stray current?

Stray currents are unwanted electrical currents that can flow through the ground or other conductive media, often affecting cathodic protection systems. Among the options provided, telluric currents represent a natural source of stray currents. These are slow, underground electrical currents that occur due to natural geological processes and variations in the Earth's magnetic field. Telluric currents are typically a result of natural phenomena, such as the movement of charged particles in the Earth, and can contribute to the complexity of corrosion issues in pipelines and other metal structures.

The other options do not serve as significant sources of stray currents. Solar radiation primarily affects surface phenomena and does not generate underground electrical currents. Air pollution can lead to corrosion through various chemical reactions but does not create stray currents. Groundwater flow, while it can conduct electricity, does not specifically generate stray currents; rather, it can be influenced by existing stray currents from other sources. Thus, telluric currents are recognized specifically for their role in generating stray currents in the context of cathodic protection and corrosion studies.