The correct answer is D) suction side of pumps. Cargo pump relief valves are typically piped to the suction side of the pumps. This is done to allow any excess pressure or flow from the pumps to be redirected back to the suction side, rather than being vented to the atmosphere or affecting other parts of the cargo system. Piping the relief valves to the suction side helps maintain system balance and prevent overpressurization. The other options are incorrect because: A) crossover lines are not the proper destination for relief valves, B) cargo pump pressure gauges should not be directly connected to the relief valves, and C) venting to the atmosphere would not be a safe or appropriate way to handle excess pressure.