which one is most ionic: A)NaCl B)MgCl2 C)KCl D)AlCl3?

In a binary compound, the greater the difference between the electronegativities of the two constitute elements, the more ionic the binary compound is.

Cl is very electronegative and K is the least electronegative metal among the four metals in the 4 options. Therefore, among the 4 options, the difference between the electronegativities of the two constitute elements in KCl is the greater, and thus KCl is the most ionic.

..... The answer is : (C) KCl

AlCl3 because it releases 3 ions