Answer: D. Chemical reactivity is determined by the outer-shell electron arrangement. Statement 2 — a full outer shell — is the only explanation of why radon is unreactive: full outer shells correspond to stable electron configurations that don't tend to gain, lose, or share electrons.
Statement 1 (non-metallic gas) describes radon's state but doesn't explain its lack of chemical reactivity. Chlorine, oxygen and nitrogen are also non-metallic gases but very reactive.
Statement 3 (even number of neutrons) is about the nucleus, not the electrons. Nuclear properties don't affect ordinary chemical reactivity.
Only statement 2 explains the unreactivity → option D (2 only).
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