Correct option is D
Coulomb friction, also known as dry friction, occurs between two bodies that are in relative motion. It is the force that opposes the relative motion of two surfaces in contact. The correct choice is Bodies having relative motion.
- Option a refers two lubricated surfaces, which involve fluid friction, not Coulomb friction.
- Option b describes the interaction between solids and liquids, which involves fluid friction.
- Option c while it is true that Coulomb friction can occur between two dry surfaces, the more general definition involves any two bodies in relative motion, which includes both dry and lubricated surfaces as long as they are in contact and moving relative to each other.
Therefore, (d) is the most accurate and general answer.
