Question

When fewer amounts of oxygen are present, carbon combines with it to give ________ gas.

Answer 1

Carbon and oxygen can combine to form two gases.

When combustion of carbon is complete, i.e. in the presence of plenty of air, the product is mainly carbon dioxide ($C{O}_2$). Sources of carbon include; coal, coke, charcoal.

When combustion of carbon is incomplete, i.e. there is a limited supply of air, only half as much oxygen adds to the carbon, and instead, you form carbon monoxide ($CO$). The relative amount of $CO$ produced depends on the efficiency of combustion.