The question is asking for the volume of 2.5 moles of oxygen at standard temperature and pressure (s.t.p), given that the molar volume of a gas at s.t.p is 22.4 dm^3.
To solve this problem, we can use the following formula:
Volume = (number of moles) x (molar volume at s.t.p)
Substituting the given values, we get:
Volume = 2.5 mol x 22.4 dm^3/mol
Volume = 56.0 dm^3
Therefore, the volume of 2.5 moles of oxygen at s.t.p is 56.0 dm^3.
So, the answer is the last option: 56.0 dm^3.