Which symptom would likely indicate a problem with a vehicle's valve stem seals?

A symptom of blue smoke at startup is often indicative of a problem with the valve stem seals. These seals control the oil that lubricates the valves and guides in the engine. When they become worn or damaged, they can allow oil to leak into the combustion chamber, especially during startup when the engine is cold, leading to blue smoke from the exhaust. This occurs because the oil is being burned along with the fuel, producing a distinct blue smoke that is noticeable during initial engine operation.

In contrast, the other symptoms listed pertain to different issues. Excessive noise from the engine may suggest a variety of mechanical problems, such as issues with the timing belt, lifters, or other engine components, rather than specifically pointing to valve stem seal failure. Frequent engine stalling can be related to fuel delivery issues, ignition problems, or a malfunctioning mass airflow sensor, rather than an issue with the valve stem seals. Fluid leakage under the vehicle could signal a range of problems, such as oil or transmission fluid leaks, and is not specifically indicative of valve stem seal problems. Thus, blue smoke at startup is the most definitive symptom associated with failing valve stem seals.