Noble gases: inert, monatomic, low density

Steps:

• Recognize Group VIII as noble gases (He, Ne, Ar, Kr, Xe), unreactive with full electron shells.
• Note all are gases at room temperature, existing as single atoms.
• Assess density: atomic masses increase down group, He lowest at 4 u.
• Eliminate options contradicting inertness or states.

Why B is correct:

• Helium's low atomic mass (4 u) versus air's average (29 u) makes it less dense, per density = mass/volume principle.

Why the others are wrong:

• A: Argon is inert and does not react with iron to form compounds.
• C: All elements from He to Xe are gases; no change to solid within this range.
• D: Noble gases are monatomic atoms, not covalent molecules.

Final answer: B