What is a common trait of smoldering combustion?

Smoldering combustion is characterized by burning slowly without a flame. This type of combustion occurs in solid materials, such as wood or charcoal, where the combustion process can sustain itself at a lower temperature and with limited oxygen. The heat generated during smoldering can be enough to maintain the reaction, but it does not produce an open flame as seen in flaming combustion.

The other options do not accurately describe smoldering combustion: burning rapidly with a flame pertains to flaming combustion, which is not the case here. Involving no smoke is also incorrect because smoldering combustion typically produces a significant amount of smoke due to the incomplete combustion of the material. Lastly, the notion that it only occurs in gas form is misleading since smoldering combustion primarily occurs in solid fuels. Thus, the defining feature of smoldering combustion is indeed its ability to burn slowly without the presence of flame.