In a shell-and-tube heat exchanger, which of the following tube types would be LEAST susceptible to tube-side fouling?

Correct Answer: Option C — Plain tubes

Plain tubes offer a smooth internal surface with no crevices or flow obstructions, which minimizes the accumulation of sediment and biological growth. While finned or corrugated tubes enhance heat transfer, they create turbulence and dead zones that significantly increase susceptibility to fouling compared to standard plain tube designs.