For one mole of a Van der Waals gas when b = 0 and T = 300 K, the PV vs 1/V plot is shown above. The value of the Van der Waals constant a (atm.L²mol⁻²) is

Vanderwaal's equation for 1 mole of gas is
(P+a/V²)(V−b)=RT
But, b=0. (given)
(P+a/V²)(V)=RT
∴PV=−a×1/V+RT
We know the basic general equation of straight line, y=mx+c
Slope=tan(π−θ)=−tan θ=−a
So, tanθ=a=21.6/0.13=1.5
Option C is correct.