The answer IS letter C. For magnesium to pair with chlorine and be neutral, magnesium must pair with two chlorines. Magnesium is in column 2 which means it has two valance electrons to "give away." If it gives away its two valance electrons, it becomes a +2 cation. Chlorine readily accepts only one electron, and when it does it becomes a -1 anion. So what does this mean? Well, Magnesium Chloride is made up of 1 Mg atom, and 2 Chlorine atoms because it forms a neutral compound. Mg gives 1 electron to each Cl atom forming a +2 -2 neutral compound. The Mg bares a +2 charge after giving its two valance electrons away, and each Cl bares a -1 charge after taking an electron. Hence +2 -2, which is neutral. This is why the formula is MgCl2.