Concept:Helium is suitable for observation balloons because it is both lighter than air and does not catch fire.
Explanation:Helium is a very light gas, with a lower density than the nitrogen and oxygen mixture in the atmosphere.
Because it is lighter than air, helium provides enough buoyancy to lift an observation balloon upward.
Helium is also chemically inert and non-flammable, so it reacts with nothing and will not burn.
If hydrogen were used, it would also lift the balloon, but hydrogen is highly combustible and dangerous.
Thus, using helium makes observation balloons float safely without the risk of explosion.
Answer:Option B: light and non-combustible.