That equal volumes of gas under the same conditions have equal numbers of molecules. this concept is a very important one, as it will be used often in future studies.

Avagadro’s law states at constant temperature and pressure, volume is directly proportional to the number of molecules of the gas.

For example  

At STP that is standard temperature and pressure at which T = 273K and P = 1 atm

Volume occupied by 1 mole ( gas molecules) of any gas is equal to 22.4L

That is

1 mole = 22.4L

2 mole = 44.8 L

3 mole = 67.2 L

So 1 mole of gas =22.4L ( gas molecules)

1 mole of gas =22.4L ( gas molecules)

1 mole of gas =22.4L ( gas molecules)

a is the correct answer