Answer: GN runs between a vertex and the midpoint of the opposite side, so it is a median.
HK is an altitude because it is perpendicular to base GJ and H is a vertex.
ML is not perpendicular to a base although it does bisect a base, but anyway, neither M nor L is a vertex: ML is neither altitude nor median.
Step-by-step explanation:
"A median of a triangle is a line segment joining a vertex to the midpoint of the opposing side, bisecting it. ... Each median divides the area of the triangle in half; hence the name" Wikipedia.
