Valence electrons in the element $\mathrm{A}$ are $3$ and that in element $\mathrm{B}$ are $6$. Most probable compound formed from $\mathrm{A}$ and $\mathrm{B}$ is

Valence electrons in $\mathrm{A}=3$

Valence electron in $\mathrm{B}=6$

Thus, $\mathrm{A}$ is electropositive and $\mathrm{B}$ is electronegative, $\mathrm{A}$ can lose three electrons and $\mathrm{B}$ can gain two electrons to attain stable configuration.

Hence, $\mathrm{A}$ exist as ${\mathrm{A}}^{3+}$ and $\mathrm{B}$ as ${\mathrm{B}}^{2-}$

$\therefore$ Compound formed is ${\mathrm{A}}_2{\mathrm{B}}_3$