Answer : The coefficient used for the oxygen in balanced chemical reaction is, 3.
Explanation :
Balanced chemical reaction : It is defined as the number of atoms of individual elements present on reactant side must be equal to the product side.
When methanol is burned in air then it react to give carbon dioxide and water as a products.
The balanced chemical reaction will be,

This reaction is a combustion reaction in which a hydrocarbon react with oxygen to given carbon dioxide and water as a products.
By stoichiometry we can say that, 2 mole of methanol react with 3 moles of oxygen to give 2 moles of carbon dioxide and 4 moles of water.
Hence, from the balanced chemical reaction we conclude that the coefficient used for the oxygen is 3.