Answer:
C. 22L
Explanation:
STP is 273 K and 1 atm 
Remember that R=0.0821 atm*L/mol*K 
Put this into ideal gas law and solve for volume 
PV=nRT → V=nRT/P 
V=[(1mol)*(0.0821 atm*L/mol*K)*(273 K)]/(1 atm) 
V=[22.4133 atm*L]/(1 atm)=22.4133 L 
So the answer is 22 L