I believe STP indicates that the pressure will be 1 atm while the temperature will be 273 K.
Therefore:
P=1 atm
V=22.4 L
R=0.08206 Latm/molK (gas constant)
Knowing that you would use PV=nRT and solve for n to get n=PV/RT
n=(1atm*22.4L)/(0.08206Latm/molK*273K)
n=0.99989 mol
That answer can be rounded to 1 so the correct answer is 1.
This can be checked by the fact that 1 mole of gas at STP is known to have a volume of 22.4L.
I hope this helps. Let me know if you have any further questions or need anything to be clarified.