What is a rich condition in the air-fuel mixture most likely to cause?

A rich condition in the air-fuel mixture occurs when there is too much fuel compared to the amount of air. This imbalance can lead to various negative consequences. When an engine runs rich, it often results in poor fuel economy because the engine is consuming more fuel than necessary to maintain effective combustion. Increased emissions are also a significant outcome, as excess fuel can lead to unburned hydrocarbons and other pollutants being emitted through the exhaust system. This excess fuel does not completely combust and produces harmful byproducts, which contributes to environmental pollution and may even cause the engine to fail emissions tests.

In contrast, the other options would not accurately reflect the effects of running a rich mixture. Increased horsepower and torque could occur under certain conditions, but typically not as a result of a consistently rich mixture, and smoother engine operation or better fuel efficiency are generally associated with a properly balanced air-fuel mixture, rather than a rich condition.