Smoke at a building fire is created by:

Smoke at a building fire is primarily created by incomplete combustion and pyrolytic decomposition. Incomplete combustion occurs when there isn't enough oxygen present for a fuel to burn completely, resulting in the production of smoke, soot, and other hazardous gases rather than just carbon dioxide and water. This scenario is common in building fires since various materials (like wood, plastics, and textiles) may not ignite completely due to limited airflow or other factors.

Pyrolytic decomposition refers to the thermal breakdown of materials when exposed to heat, causing them to release flammable gases before actual ignition occurs. This process is particularly relevant for synthetic materials and certain types of wood during a fire, significantly contributing to the formation of smoke.

Thus, both incomplete combustion and pyrolytic decomposition are crucial in understanding why smoke is generated during a fire, making this option the most accurate.