A. N2 Let's look at the substances and see which would be likely to be a near ideal gas at STP. A. N2 * Nitrogen gas. It's a gas at 273.15 K and is fairly non-reactive. This is a very good candidate for the correct answer. B. H2O * Water. There is some H2O in gaseous form, but it's more likely to be a solid or liquid. This isn't a good candidate, so it's the wrong answer. C. CH3OH * Methanol. Fairly large molecule and has a boiling point well above that of 273.15 K. So it's more likely to be a liquid. So also the wrong answer. D. HF * Hydrogen fluoride. Not a nice chemical to deal with. And a boiling point of 340.25 K which is much higher than the 273.15 of STP. Not the right answer. Of the 4 candidates, only N2 has a boiling point well below the 273.15 K of STP and is the only substance listed that will be completely a gas at STP. So that's the correct answer.