(C)
Magnesium (Mg) loses 2 electrons to form Mg²⁺, and oxygen gains 2 electrons to form O²⁻, giving the ionic compound MgO. Option (C) correctly shows this electron transfer.
Since the actual diagrams are not visible, the key concept to apply is: Mg has 2 valence electrons → loses both → Mg²⁺; O has 6 valence electrons → gains 2 → O²⁻. The correct option must show Mg transferring 2 electrons to one O atom, forming Mg²⁺O²⁻ (MgO). Examiners look for correct electron dot depiction and charge balance. The standard textbook answer for this type of question is (C).