Final answer:
The best observation supporting helium's classification as a noble gas is its full outer electron shell, indicating stability and chemical inertness, which are characteristic of noble gases.
Step-by-step explanation:
The observation that would best support the conclusion that helium should be classified as a noble gas is that helium has a full outer electron shell. Noble gases like helium, neon, argon, etc. are known for having filled outer subshells, which makes them particularly stable. This stability is evidenced by their large ionization energies and their lack of reactivity as they do not readily give up electrons. In particular, helium has a full outer shell of two s-electrons, making it chemically non-reactive and unable to form compounds easily at room temperature and pressure. This chemical inertness is a characteristic property of noble gases.