Which rigid cylinder contains the same number of gas molecules at stp as a 2.0-liter rigid cylinder containing h2(g) at stp?
(1) 1.0-l cylinder of o2(g)
(2) 2.0-l cylinder of ch4(g)
(3) 1.5-l cylinder of nh3(g)
(4) 4.0-l cylinder of he(g)

When we say STP that means the system is at its standard temperature and pressure. Both systems is having the same condition hence temperature and pressure is constant. We then set the amount of gas in both cases to be equal. Hence from the ideal gas law, 
PV=nRT we can say that with everything constant, volume must also be constant. 

The answer is (2) 2 L of methane gas.