Final answer:
Valves close off the intake and exhaust ports for each engine cylinder, with A) Ports being the correct answer. The intake valves allow air in, while exhaust valves let combustion byproducts out during different phases of the engine cycle. Option A is correct.
Step-by-step explanation:
Valves close off the intake and exhaust ports for each engine cylinder. The correct answer to your question is A) Ports. Engine valves play a critical role in controlling the entry and exit of air and exhaust gases in the internal combustion engine.
The intake valves open to allow the air-fuel mixture to enter the cylinder, and then close during the compression and combustion processes. After combustion, the exhaust valves open to allow the byproducts of combustion to leave the cylinder, going through the exhaust ports.
During the constant-volume exhaust phase of the engine cycle, when the exhaust valve opens, some of the combustion products escape, but since there is almost no movement of the piston, the volume in the cylinder remains constant. The exhaust valve is also involved in the isobaric compression phase, where it remains open to drive out the remaining combustion products.