How can i balance a combustion reaction?

Exemple 

In combustao of methane ( CH4 ) 

CH4 + O2 = CO2 + H2O

C is balanced

To balance H multiply  H2O on the right  by 2

CH4 + O2 = CO2 + 2 H2O

To balance O multiply O2 on the right by 2

CH4 + 2 O2  = CO2 + 2 H2O

Hope this helps!

Man i dont know this answer

The answer is d.potassium