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.

the electronic configuration of an atom with eight electrons is 1s²2s²2p⁴. the orbital diagrams are shown in the figure.

as per the hunds rule:

1. electrons tend to avoid being in the same orbital as per as practicable.

2. electrons in different orbitals of the same energy will have their spin parallel.

this can be illustrated from the multiplicity of the orbitals of the atom. as there are four electrons in the p orbital and p orbital has 3 sub-shells. the electrons can be fed in different ways as shown in the figure but the correct orbital diagram will be as per the hunds rule.