The correct answer is A) during the valve overlap period. Clearance volume scavenging in a turbocharged, four-stroke cycle diesel engine is accomplished during the valve overlap period, when both the intake and exhaust valves are open briefly. This allows the high-pressure exhaust gases from the previous cycle to be expelled from the cylinder, while the incoming fresh air charge is drawn in, improving the efficiency of the engine. The other options are incorrect because: B) with only the exhaust valve open would not allow the fresh air charge to enter the cylinder; C) at a pressure below atmospheric is not necessary for clearance volume scavenging; and D) without cooling the cylinders or pistons is not relevant to the scavenging process.