The correct option is A 1
Balancing of a chemical reaction emphasises equal amount of atoms on the reactants side and products side. In the reaction (ii), there are two oxygen atoms on the right hand side. To balance this, coefficient of O2 must be equal to 1 (one). So, X = 1.
Complete combustion of carbon results in the formation of carbon dioxide (CO2) whereas the formation of carbon monoxide (CO) takes place due to incomplete combustion. This is due to the insufficient amount of air (oxygen) which leads to incomplete combustion.