The correct answer is B) cracked cylinder head. A cracked cylinder head can allow combustion gases to escape into the expansion tank, causing the venting of gases through the tank. This is because a cracked cylinder head compromises the seal between the combustion chamber and the cooling system, allowing pressurized gases to enter the cooling system. The other options are incorrect because: A) A leaking oil cooler would not cause combustion gases to vent through the expansion tank. C) Worn piston rings may cause increased blow-by, but this would not directly result in combustion gases entering the cooling system. D) A leaking exhaust valve would allow combustion gases to escape through the exhaust system, not the expansion tank.