Answer : The correct option is, (4) [tex](H_2O)[/tex]
Explanation :
Combustion reaction : It is a reaction in which a hydrocarbon react with the oxygen gas to produces carbon dioxide as water as a products.
The given incomplete equation are :
[tex]2C_2H_6+7O_2\rightarrow 4CO_2+6_[/tex]
In this equation, we see that a hydrocarbon and oxygen gas are present on reactant side and carbon dioxide are present on product side but water molecules are missing on the product side. So, we are adding water [tex](H_2O)[/tex] on missing side.
The given complete equation will be :
[tex]2C_2H_6+7O_2\rightarrow 4CO_2+6H_2O[/tex]
Therefore, the formula missing on product is, [tex](H_2O)[/tex]