Magnesium would acquire a stable arrangement of electrons by bonding with one atom of which of the following elements?

oxygen

chlorine

helium

nitrogen

Here we have to choose the anion from the given atom, which will combine with Mg in a stable arrangement as a monovalent anion.

Mg²⁺ will only combine with one single anion O²⁻ to form MgO.

The electronic configuration of Magnesium (Mg¹²) is 1s²2s²2p⁶3s², so the most stable ionic form of Mg is Mg²⁺ having inert gas electronic configuration i. e. 10 (Neon).

Now for a single anion the charge of the ion should be - 2. Among the given atoms the electronic configuration of oxygen (O⁸) is 1s²2s²2p⁴, so the divalent anion of oxygen posses 10 electrons, inert gas electronic configuration (Ne).

The electronic configuration of chlorine (Cl¹⁷) is 1s²2s²2p⁶3s²3p⁵. Thus the monovalent anion will have 18 electrons i. e. inert gas electronic configuration, Argon (Ar).

The helium has only 2 electrons and it cannot leave the electrons as it will be highly unstable in that condition.

The nitrogen (N⁷) 1s²2s²2p³, thus the divalent anion will poss 9 electrons which is not stable.

Thus Mg²⁺ will only combine with one single anion O²⁻ to form MgO, which will be highly stable.