\(\frac{{dP}}{{dV}} = - \gamma \frac{P}{V}\)==> |Slope| \(\propto \gamma\)
From the given graph \((Slope)_2 > (Slope)_1\) ==> \({\gamma _2} > {\gamma _1}\)
therefore \(1\) should correspond to \(O_2 (\gamma = 1.4)\) and \(2\) should correspond to \(He\) (\(\gamma = 1.66\))