Which issue is indicated by worn spark plugs if a vehicle's smog test results in HC of 160 ppm?

Worn spark plugs can lead to incomplete combustion of fuel within the engine, which results in higher hydrocarbon (HC) emissions. Hydrocarbons are unburned or partially burned fuel that escapes into the exhaust system, contributing to smog and air pollution. When the smog test shows HC levels at 160 ppm, it indicates excessive emissions, reflecting the impact of the worn spark plugs on combustion efficiency.

In contrast, options like increased fuel efficiency, improved acceleration, and stable engine performance do not align with the effects of worn spark plugs. Worn plugs typically cause misfires or rough idling, leading to decreased performance and efficiency, not improvements. Hence, the correct focus on excessive emissions directly ties back to how worn spark plugs affect engine performance and emissions testing results.