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

Valence electron in $A=3$ Valence electron in $B=6$ Thus, $A$ is electropositive and $B$ is electronegative. A can lose three electrons and $B$ can gain two electrons to attain stable configuration. Thus, $A$ exists as $A^{3+}$ and $B$ as $B^{2-}$ thus, compound is $A_2 B_3$.