Q.Most easily liquefiable gas is

Step 1. Recall the boiling-point trend down Group 18.

Boiling point rises steadily down the noble-gas group as atomic size and polarisability increase, strengthening the weak instantaneous-dipole (van der Waals/London dispersion) forces between atoms: He (4.2 K) < Ne (27.1 K) < Ar (87.3 K) < Kr (119.7 K) < Xe (165.1 K) < Rn (211.5 K).

Step 2. Relate boiling point to ease of liquefaction.

A gas with a higher boiling point condenses to a liquid at a higher (easier-to-reach) temperature, and generally under milder conditions overall, than a gas with a very low boiling point that requires extreme cooling.

Step 3. Compare the given options. …

Rank the candidate gases by boiling point (which rises with atomic size/polarisability down Group 18) and recognise th …

Assuming the smallest, lightest noble gas (helium) liquefies most easily because it seems the 'simplest' — in reality helium's very weak …