What type of flame is most efficient in burning fuels completely?

The most efficient type of flame in terms of completely burning fuels is the diffusion flame. A diffusion flame occurs when fuel and oxidizer (usually air) mix and react in a combustion process that occurs throughout the flame's volume. This type of flame allows for adequate mixing of fuel and oxygen, resulting in a more complete combustion. The efficiency of a diffusion flame is notable since it minimizes unburned fuel and reduces the formation of combustion byproducts, such as soot or carbon monoxide, which are common in incomplete combustion scenarios.

In contrast, other types like stack flames or depletion flames do not facilitate the same level of effective mixing or combustion. Stack flames typically arise in restricted conditions, leading to partial combustion, while depletion flames may occur when the fuel supply diminishes and combustion cannot be sustained at an optimal level. Therefore, diffusion flames are recognized for their ability to achieve greater combustion efficiency, making them the correct answer in this context.