P-V plots for two gases during adiabatic process are shown in the figure. Plots 1 and 2 should correspond respectively to:

Slope of adiabatic curve ∝ γ ∝ $\frac{1}{ Atomicity of the gas}$

So, γ is inversely proportional to atomicity of the gas. [As γ = 1.66 for monoatomic gas, γ = 1.4 for diatomic gas and γ = 1.33 for triatomic non-linear gas.] From the graph it is clear that slope of the curve 1 is less so this should be adiabatic curve for diatomic gas (i.e. O₂). Similarly slope of the curve 2 is more so it should be adiabatic curve for monoatomic gas (i.e. He).