What type of soot residue results from a malfunction in an oil-fired heating system?

The correct answer is based on the characteristics of the residue created during a malfunction in an oil-fired heating system. Puff back refers specifically to a situation where unburned fuel is expelled from the combustion chamber, often due to a malfunction in the heating system, such as a blocked flue or an improper air-fuel mixture. This expulsion creates a distinct type of soot that is typically more greasy and can lead to significant staining and residue accumulation on surfaces.

In contrast, other types of soot residues, such as protein residue, originate primarily from cooking fires and the combustion of food items, while plastic residue arises from the burning of synthetic materials, which also produces a unique residue pattern. Incomplete combustion residue can occur in a variety of fire scenarios but does not specifically pertain to oil-fired heating system failures. Thus, the characteristics of puff back make it the most appropriate answer regarding soot residue resulting from such mechanical malfunctions.