A 2-4 heat exchanger involves

ANS:C - both counter and parallel-flow of the fluids.

A 2-4 heat exchanger typically involves both counter-flow and parallel-flow of the fluids. In a 2-4 heat exchanger, one fluid flows through tubes while the other flows through the shell surrounding the tubes. This configuration allows for both counter-flow and parallel-flow arrangements, depending on the direction of flow of the fluids within the tubes and the shell. Regarding the pressure drop, it is not directly related to whether the heat exchanger is a 1-2 or 2-4 exchanger. The pressure drop depends on various factors such as the flow rates, fluid properties, geometry, and design of the heat exchanger. Therefore, it cannot be generalized that a 2-4 exchanger would have a smaller pressure drop compared to a 1-2 exchanger. So, the correct answer is: "both counter and parallel-flow of the fluids."