To what extent must the oxygen content be reduced before an explosion of methane and air is impossible?: (A) 12 percent, (B) 10 percent, (C) 11 percent, (D) 7 percent
Answer is A. 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.