ANS:C - heat transfer area.

The main purpose of providing fins on a heat transfer surface is to increase the heat transfer area. Explanation:

• Fins are extended surfaces attached to the primary heat transfer surface to enhance heat transfer.
• Fins increase the surface area available for heat transfer between the solid surface and the surrounding fluid (or medium).
• By increasing the surface area, fins facilitate greater heat transfer rates for a given temperature difference between the surface and the surrounding medium.
• Fins are particularly useful in situations where the heat transfer coefficient between the primary surface and the fluid is low, as they help compensate for this by providing additional surface area for heat exchange.
• While fins may indirectly affect the temperature gradient and heat transfer coefficient, their primary function is to increase the heat transfer area, which ultimately enhances the overall heat transfer performance of the system.