Helium is used in observation of ballons because it is?

Analyze the question: The question asks why helium is used to fill (observation/weather) balloons. We need to match the property of helium that makes it suitable.

Concept: For a balloon to rise, the gas inside must be less dense (lighter) than air. For it to be safe, the gas should not catch fire or explode.

Step-by-step solution:

1. Density: Helium is a noble gas with a relative atomic mass of 4. Air has an average molar mass of about 29. Since helium is much less dense than air, it provides lift. So helium is light. This eliminates options C and D, which say "heavy".

2. Combustibility: Helium is a noble gas (Group 18/0). Its outer shell is complete (2 electrons), so it is chemically inert. It does not burn or support combustion. So helium is non-combustible. This eliminates option A.

3. Comparison with hydrogen: Hydrogen is also light, but it is highly flammable (it can explode), so it is dangerous for balloons. Helium is the safer choice.

Answer: Helium is light and non-combustible, which is option B.

Exam tip: Remember "Helium = Harmless and Handy": it is light enough to float and inert enough to be safe. Whenever you see a question about balloons or airships, think "low density + non-flammable". Also note that noble gases are generally unreactive because of their full outer shells.