Carbon monoxide is formed when fuels are burned:

with sulfur impurities

without enough oxygen

at high temperatures

that contain lead

Without enough oxygen.

Step-by-step explanation:

In the presence of excess oxygen, the combustion of fuel produces carbon dioxide. For example, propane burns according to the equation

C₃H₈ + 5O₂ ⟶ 3CO₂ + 4H₂O

In the presence of a limited amount of oxygen, carbon monoxide is also formed.

C₃H₈ + ⁷/₂O₂ ⟶ 3CO + 4H₂O

The process is called incomplete combustion.

(a) is wrong. Sulfur impurities cause the formation of sulfur dioxide.

(c) is wrong. High temperatures usually lead to more complete combustion.

(d) is wrong. Lead compounds inhibit knocking in gasoline engines, but they do not affect the formation of carbon monoxide.

Without enough oxygen.