You can rule out both EGF and GEF because they are representing another segment of the graph.
The answer should be DGE. If you use DGE to draw a triangle starting from the first letter (D) and then go to E and then G, then E will be in the middle.
I was taught that the middle letter of any sequence of letters is the angle of said sequence because line DG and line GE connect to form it.
