The correct answer is A) Open a COW nozzle forward and one aft and drain the line into the after tank by gravity. This is the correct action because after completing a crude oil wash (COW), the lines running to the washing machines should be drained to remove any residual oil. Draining the lines by gravity through the COW nozzles is the preferred method, as it avoids the need to use pumps or compressed air which could potentially create a hazardous situation. The other options are incorrect because: B) Using the supply line pump to pull a suction could create a vacuum and potentially suck in air, C) Leaving the oil in the line primed for the next COW is not recommended, and D) Blowing out the line with compressed air could create a risk of ignition or explosion.