Which of the following gas is lightest

$\begin{array}{1 1}(a)\;\text{Dihydrogen}\\(b)\;\text{Helium}\\(c)\;\text{Dinitrogen}\\(d)\;\text{Dioxygen}\end{array}$

Among the given gases $H_2$ is lightest (V.D = 1)
Molecular weight of hydrogen is less.
Hence (a) is the correct answer.
edited Nov 18, 2017