Bond order of the covalent molecule, $\mathrm{AX}$ is given as $2.5$, As $\mathrm{AX}$  is a covalent diatomic molecule of second row elements of periodic table we can consider $\mathrm{CO}, \mathrm{NO}$  but bond order of $\mathrm{CO}$ having $14$  electrons which is isoelectronic to nitrogen should be equal to $3$ and $\mathrm{NO}$  having 15 electrons will have a bond order of $2.5$.

Molecular orbital configuration and bond order calculation of $\mathrm{NO}$ is

ABMO$=$ antibonding molecular orbital

Bond order $=$Number of bonding electrons $-$ number of anti bonding electrons.

Note: Total number of electrons equal to 13 will also have the 2.5 bond order. But in this case neutral diatomic molecule will not be possible