What best defines a noble gas?

• A. Highly un-reactive, colorless gases
• B. Gases that readily form compounds with hydrogen
• C. Elements that have a full d-subshell
• D. Reactive metals in group 1

Answer: (A)

Noble gases are defined by having a complete outer electron shell, which makes them highly stable and chemically inert. This full valence shell means they have little tendency to gain, lose, or share electrons, so they appear as colorless, monoatomic gases at room temperature and rarely form compounds. That inertness is the hallmark, which is why the description "highly un-reactive, colorless gases" best fits them.

The other statements don’t fit as a general rule: noble gases don’t readily form compounds with hydrogen, they don’t rely on a filled d-subshell (their outer electrons are in s and p orbitals), and they are not reactive metals in group 1.