What is a common characteristic of materials classified as combustibles?

Materials classified as combustibles share the characteristic that they can all undergo pyrolysis. Pyrolysis is the process in which organic materials decompose under high heat in the absence of oxygen, breaking down into simpler chemical compounds and producing flammable vapors. This is a crucial aspect of how combustibles behave when exposed to heat; they not only ignite but also generate gases that can combust, contributing to fire spread.

In contrast, the other options do not accurately reflect the typical characteristics of combustible materials. Materials that do not ignite easily are generally considered non-combustible or would fall under specific classifications of fire-resistant materials. Retaining heat effectively refers to materials' thermal properties but does not signify their flammability or combustibility. Lastly, being nonflammable directly contradicts the nature of combustibles, which are defined by their ability to catch fire under the right conditions. Therefore, the ability to undergo pyrolysis is a defining and common attribute of combustible materials.