ANS:D - decreases.

When the baffle spacing in a shell and tube heat exchanger increases, it typically leads to an increase in the Reynolds number of the shell side fluid. Baffle spacing affects the flow regime within the shell side of the heat exchanger. With larger baffle spacing, the flow area available for the fluid to pass through increases, which often results in higher fluid velocities. Higher fluid velocities lead to an increase in the Reynolds number, which is a dimensionless parameter representing the ratio of inertial forces to viscous forces in the fluid flow. Therefore, the correct option is: increases.