$$P + \frac{a}{V^2}(V - b) = RT$$ represents the equation of state of some gases. Where $$P$$ is the pressure, $$V$$ is the volume, $$T$$ is the temperature and $$a, b, R$$ are the constants. The physical quantity, which has dimensional formula as that of $$\frac{b^2}{a}$$, will be:

We are given the equation of state $$\left(P + \frac{a}{V^2}\right)(V - b) = RT$$.

Since $$b$$ is subtracted from $$V$$, it must have the same dimensions as volume, so $$[b] = [V] = L^3$$.

Now, since $$\frac{a}{V^2}$$ is added to $$P$$, it must have the same dimensions as pressure. This gives us

$$[a] = [P][V^2] = ML^{-1}T^{-2} \times L^6 = ML^5T^{-2}$$

We can now find the dimensional formula of $$\frac{b^2}{a}$$:

$$\left[\frac{b^2}{a}\right] = \frac{L^6}{ML^5T^{-2}} = M^{-1}L^1T^{2}$$

This is the dimensional formula of the reciprocal of pressure (since compressibility is defined as $$\frac{1}{\text{Bulk modulus}}$$ and has dimensions $$M^{-1}LT^2$$).

Hence, the correct answer is Compressibility.