The correct answer is C) raise the pressure in the system above atmospheric. To use standard leak detection methods on a low-pressure refrigeration system charged with refrigerant, the pressure in the system must be raised above atmospheric pressure. This is because standard leak detection methods, such as using a halide torch or electronic leak detector, require the system to be under positive pressure to effectively identify any leaks. If the system is under low or negative pressure, the refrigerant will not be able to escape the system and be detected by the leak detection methods. The other options are incorrect because lowering the pressure below atmospheric (A) would not allow the refrigerant to escape for detection, and adding nitrogen (B) or cooling the refrigerant (D) would not directly raise the system pressure to the required level for effective leak detection.