The correct answer is Argon. By volume, dry air is roughly 78% nitrogen, 21% oxygen, about 0.93% argon, and around 0.04% carbon dioxide, with traces of neon, helium, methane, and other gases. Among the gases listed, argon is the one that remains largely unreactive.

The reason lies in its electronic structure. Argon is a noble gas belonging to Group 18 of the periodic table. Its outermost electron shell is completely filled (a stable octet configuration), so it has no tendency to gain, lose, or share electrons. Because chemical bonding is driven by the urge of atoms to achieve a stable electron arrangement, an atom that already possesses one has essentially no driving force to react. As a result argon forms no stable compounds under ordinary atmospheric conditions and takes no part in atmospheric chemical reactions.

Why the other options are incorrect: Oxygen is highly reactive — it supports combustion, causes rusting of metals, and is essential to respiration. Carbon dioxide is chemically active, dissolving in water to form carbonic acid and being consumed by plants during photosynthesis. Methane is a combustible hydrocarbon that reacts readily with oxygen and other species and is an active greenhouse gas. Although nitrogen (not offered here) is fairly inert, it still forms compounds such as ammonia and nitrates under high-energy conditions, whereas argon remains inert even then. Argon is therefore the least reactive of the choices.