I didn't read through the entire proof but it appears the obvious answer by deduction is D: using the point slope formula.
The proof is about finding the slope of AE BF and CD, so midpoints have nothing to do with it.
It's also not the definition of a slope and it also doesn't have to do with parallel lines because it mentions none of it in previous statements.
Thus the answer is using the point-slope formula as the previous statement was entirely dedicated to finding the coordinates for the points. All statement 7 did was to plug those values in to get the slope.
