Final answer:
Without the proper air filter, particles and contaminants can cause wear to engine components, with piston rings being most likely to be damaged due to their role in maintaining compression and the oil seal within the engine.
Step-by-step explanation:
If a vehicle has been run without the proper air filter, the part that would most likely be damaged is the piston ring. An air filter is critical for removing particles and contaminants from the air before it enters the engine's combustion chamber.
Without a filter, these particles can enter the engine, causing abrasive wear to the surfaces of engine components including the piston, cylinder walls, and piston rings. This wear can lead to damaged piston rings, which are essential for maintaining compression and the oil seal within the engine. Additionally, a compromised piston ring can result in reduced engine performance, increased oil consumption, and potentially severe engine damage over time.