Answer:
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.
