To what extent must the oxygen content be reduced before an explosion of methane and air is impossible?
12 percent
10 percent
11 percent
7 percent
For a mixture of strictly methane and air, the oxygen content must be 12 percent or less. 
It is however, significant to note that there is a combination of combustible gases following explosions and during fires.
So the maximum allowable oxygen will vary according to variations in the combustible and inert gases.