1. Use van der Waals equation for one mole

For one mole, the van der Waals equation is

$\left(P + \frac{a}{V^2}\right)(V-b)=RT$

Given $b=0$, so it becomes

$\left(P + \frac{a}{V^2}\right)V = RT$

$PV + \frac{a}{V} = RT$

Therefore,

$PV = RT - a\left(\frac{1}{V}\right)$

This is the equation of a straight line when we plot $PV$ versus $\dfrac{1}{V}$.

2. Compare with straight line form

The line is of the form

$y = c + mx$

with

• $y = PV$
• $x = \dfrac{1}{V}$
• intercept $= RT$
• slope $= -a$

So, from the graph:

• the y-intercept should be $RT$
• the slope gives $-a$

3. Find intercept from temperature

Given $T=300\,\text{K}$ and for one mole,

$RT = (0.0821)(300) \approx 24.6\,\text{atm L}$

So the graph should intercept the $PV$-axis near $24.6$.

4. Read slope from the graph

From the given straight-line plot, the decrease in $PV$ per unit increase in $1/V$ is $1.5$.

Hence,

$\text{slope} = -a = -1.5$

So,

$a = 1.5\,\text{atm L}^2\text{ mol}^{-2}$

5. Match with options

Thus the correct option is

$\boxed{1.5}$

which is Option C.