Final answer:
Carbon monoxide is primarily formed as a result of the partial combustion of fossil fuels option(4) when there is not enough oxygen present to produce carbon dioxide.
Step-by-step explanation:
Carbon monoxide is formed as a product of partial combustion of fossil fuels. When there is insufficient oxygen during the combustion process, carbon monoxide (CO) is produced instead of carbon dioxide (CO2). This incomplete combustion can happen in various settings, including domestic environments with faulty or unvented heaters and motor vehicles without efficient catalytic converters.
Industrially, carbon monoxide is also produced in processes such as passing air over excess carbon at high temperatures, where it can be used as a precursor for chemical synthesis, like acetic acid production.