Equilibrium: JEE Main 2025 Chemistry question

At temperature $T$, compound $\mathrm{AB_{2(g)}}$ dissociates as $\mathrm{AB_{2(g)} \rightleftharpoons AB_{(g)} + \frac{1}{2}B_{2(g)}}$ having degree of dissociation $x$ (small compared to unity). The correct expression for $x$ in terms of $K_p$ and $p$ is