What characteristic identifies a properly functioning combustion flame?

A properly functioning combustion flame is characterized by its stable blue color. This blue flame indicates that the combustion process is efficient, with a well-mixed fuel and oxidizer, which is typically air. The blue color is a result of complete combustion, signifying that the gas is being burned cleanly and efficiently, thus minimizing the production of harmful byproducts like carbon monoxide and unburnt hydrocarbons.

A stable blue flame also indicates that there is an adequate supply of oxygen, which is critical for achieving optimal combustion. In contrast, a yellow flame often points to incomplete combustion, leading to a less efficient burn that can produce soot and potentially hazardous emissions.

Moreover, the absence of excessive smoke and visible light, as indicated in the other options, is associated with inefficient or incomplete combustion. A properly functioning flame should not produce significant smoke, which is a sign of poor combustion conditions. Therefore, the blue, stable flame is the hallmark of a properly functioning system, ensuring safety and optimal performance in gas appliances.