In the equation [X + a Y ^ 2 ] [ Y - b ] = RT, X is pressure, Y is...

$X \text{ and } \dfrac{a}{{Y}^{2}}$ have the same dimensions. $Y \text{ and } b$ also have the same dimensions.

Therefore,

$\dfrac{a}{{Y}^{2}} = M{L}^{- 1}{T}^{- 2} ⇒ a = M{L}^{- 1}{T}^{- 2} \times {L}^{6} = M{L}^{5}{T}^{- 2}$

and $b = {L}^{3}$.

So, the dimensions of $\dfrac{a}{b}$ is,

$\dfrac{a}{b} = \dfrac{M{L}^{5}{T}^{- 2}}{{L}^{3}} = M{L}^{2}{T}^{- 2}$.

This is the dimensional form of energy. Hence, this is the right option.