The correct answer is A) Length of the piston skirt. The length of the piston skirt is a key design feature that prevents the air under the pistons from being pumped back through the scavenge ports during the piston power stroke in a loop or cross scavenged engine. The longer piston skirt acts as a barrier, blocking the flow of air back through the scavenge ports and ensuring that the scavenging air is directed out of the cylinder as intended. The other options are incorrect because: B) Masked intake ports do not prevent backflow through the scavenge ports, C) Lower liner seals are not the primary mechanism for this, and D) Positive pressure from the blower alone is not enough to prevent backflow without the piston skirt design.