At $300\mathrm{K}$ and the same moderate pressure, the compressibility factor of $\mathrm{He}$ is slightly greater than 1 while that of $\mathrm{NH}{}_3$ is well below 1. The best explanation is

1. AHe has a larger $a$ and NH3 a larger $b$
2. BFor He the repulsive (excluded volume) term $b$ outweighs its tiny attraction term, while for NH3 strong hydrogen bonding makes $a/RT$ dominate
3. CHe is above its critical temperature and NH3 is below it, so NH3 is a liquid
4. DHe has more degrees of freedom than NH3