Answer:
MgCl2
Explanation:
Lattice energy will be higher on ion with higher charge, or smaller size. A higher charge means it will have a stronger force. Smaller size ion will have less distance.
Magnesium has +2 charge and located in the third row, make it strongest of these three. Natrium will be the second strongest even though its charge is +1, the same as Kalium. Natrium has smaller size as it is on the third row, compared to Kalium that was in the fourth row.
