A2B → 2A+ + B2−, B2− has a complete octet in its di-anionic form, thus in its atomic state it has 6 electrons in its valence shell. As it has a negative charge, it has acquired two electrons to complete its octet.
For the reaction, $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$
Attainment of equilibrium is predicted correctly by: