Given below are the critical temperatures of some of the gases :
 

Gas	Critical temperature (K)
He	5.2
CH4	190
CO2	304.2
NH3	405.5

The gas showing least adsorption on a definite amount of charcoal is :

The amount of gas adsorbed by a solid depends on the nature of the gas. In general, easily liquefiable gases are easilyadsorbed.
Here easily liquefiable means the gases with higher critical temperatures are readily adsorbed as van der Waal’s forces arestronger.
As He has the least critical temperature of all the available options, He is least adsorbed on charcoal.