Final answer:
Wooden structures in houses act as fuel for fires, unlike steel framing, concrete walls, and fiber cement siding, which are less combustible and fire-resistant.
Step-by-step explanation:
The question asks which aspect of houses makes them a source of fuel for fires. Among the options provided, c. Wooden structures are known to be combustible and can therefore act as fuel for fires, contributing to their spread. In contrast, steel framing and concrete walls are less likely to contribute to fire spread due to their non-combustible nature. Fiber cement siding is also designed to be fire-resistant. Historical events have shown that cities have transitioned from wooden structures to materials less likely to fuel fires, such as brick and steel, as a lesson from catastrophic fires.