A restricted exhaust system will primarily cause what issue?

A restricted exhaust system leads to poor engine performance primarily because it hampers the ability of exhaust gases to exit the engine efficiently. When the exhaust flow is restricted, back pressure increases, which can lead to a reduction in engine power and overall efficiency. The engine struggles to push exhaust out, which can restrict the intake of fresh air and fuel for the next combustion cycle, further compounding performance issues.

While it's true that a restricted exhaust can also lead to increased carbon monoxide (CO) emissions due to incomplete combustion, the most immediate and noticeable impact is the degradation of engine performance, making option B the best choice. The performance is compromised due to the engine's inability to expel exhaust gases properly, directly affecting power output and response during operation.