On which type of fire should you not use a B:C fire extinguisher?

A B:C fire extinguisher is specifically designed for use on electrical fires and flammable liquid fires, as indicated by the classification. The "B" designation indicates that it can be used for fires involving flammable liquids, such as gasoline or oil, while the "C" designation indicates its effectiveness on electrical fires.

Using a B:C fire extinguisher on wood fires is not advisable because wood fires fall under the classification of Class A fires. Class A fires involve ordinary combustible materials like wood, paper, or textiles. The extinguishing agents in a B:C fire extinguisher are not effective against these types of materials and might not adequately suppress the flames, potentially leading to a less effective fire response.

In summary, while a B:C fire extinguisher is suitable for specific fire types, it is not appropriate for wood fires, making this the correct choice in the context of the question.