Regarding fouling in a heat exchanger, which is the correct description of its effect on pressure drop?

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Fouling increases flow resistance in a heat exchanger. The deposits on tube walls and inside passages reduce the effective flow area and roughen the surface, so the fluid faces more friction and constriction as it moves through. For the same flow rate, this higher friction and smaller passage cross-section require more pressure to push the fluid through, raising the pressure drop across the exchanger. The other options don’t describe this mechanical effect: color change isn’t related to pressure loss, and while fouling can reduce heat transfer efficiency, it doesn’t inherently decrease the fluid’s temperature in a way that defines pressure drop. Saying there’s no effect on pressure drop is incorrect.