Q. The lightest gas is

 1711  219 VMMC MedicalVMMC Medical 2007 Report Error

Solution:

Key Idea: The gas with lowest atomic mass is lightest gas. The atomic mass of given choices are as follows atomic mass = 14.006 Ar atomic mass = 39.948 Rn atomic mass = 222.0 He atomic mass =4.003 He is the lightest gas.