How many moles of oxygen will occupy a volume of 25 l at 1.2 atm and 25 c?

Use the formula PV=nRT
convert T into kelvin by 25+273
put in the values given and calculate n

You have to use the ideal gas law which is PV=nRT
P: pressure of the gas (in this case 1.2 atm)
V: volume of the gas (in this case 25 L)
T: Temperature of the gas (in this case 25 °C which is 298K)
  (You need to convert all temperature values to Kelvin when using gas laws)
n: is the number of moles (in this case the unknown)
R: the gas constant which is 0.0821Latm/molK
rewrite the equation so that n=PV/RT than plug in all of the values.
You should get n=1.2262mol 

Therefore the answer is 1.23 moles of oxygen.

I hope this helps.  Let me know if anything is unclear.

The correct answer is b