Question

?

Answer 1

Because lattices are formed by ionic bonds and Mg and N create an ionic bond>

Answer 2

Mg doesn't make Mg2-, it makes Mg2+ so C cannot be it.

Answer 3

using process of elimination

D can't be right either (Mg loses 2 not 3)
Mg and N combine to make a more stable lattice, so it's unlikely that it would be endothermic (it should release energy rather than absorb it)

so B would be my best bet