If the measure of N were 22°, then N and L are the congruent base angles of the isosceles triangle. This means , which is clearly not the case from the figure.
Incorrect
If the measure of N were 68°, the measure of M would also be 68° and the angle sum for MLN would be 68° + 68° + 22° = 158°. But the angle sum must be 180°.
Incorrect
If the measure of N were 78°, the measure of M would also be 78° and the angle sum for MLN would be 78° + 78° + 22° = 178°. But the angle sum must be 180°.
Correct!
The correct answer is D.
From the figure, , so MN as base angles of an isosceles triangle. Then, 22° + 2m(N) = 180°; m(N) = = 79°.
Incorrect
If the measure of N were 89°, the measure of M would also be 89° and the angle sum for MLN would be 89° + 89° + 22° = 200°. But the angle sum must be 180°.
N ?
, which is clearly not the case from the figure.
MLN would be 68° + 68° + 22° = 158°. But the angle sum must be 180°.
, so 
= 79°.