Which sign might specifically indicate a problem with the engine's emissions system?

The illuminated check engine light serves as a critical indicator that there may be an issue with the vehicle's emissions system. This warning light can be triggered by various factors related to emissions, including problems with the oxygen sensors, catalytic converter, or exhaust gas recirculation system. When the emissions system is not functioning properly, it can lead to increased vehicle emissions, which is why the check engine light is a standardized alert for drivers to seek further diagnosis.

In contrast, the other options point to different types of mechanical or performance issues. For instance, vehicle shaking during acceleration often pertains to issues with the engine, transmission, or fuel system rather than emissions directly. Noisy brakes typically indicate problems with brake pads or rotors, and misalignment of tires relates to suspension and steering components. Each of these issues requires attention but does not specifically indicate emission system concerns as directly as the check engine light does.