Rank the vector combinations on the basis of their magnitude.
rank from largest to smallest. to rank items as equivalent, overlap them.
(f+c), (d), (a+e), (a+c), (a+d), (a+b)

In order to answer this, we need to know the magnitude of each one. It would really help if you'd tell us that, or at least tell us the locations of points A, B, C, D, E, and F, and we'll calculate the magnitudes. Right now, we have nothing at all to work with.