Given that ray FE forms a right angle EFB, we know that angle EFB measures 90 degrees.
Since lines AB and CD intersect at point F, angle EFC and angle EFD are adjacent angles that form a straight line (along line CD). Therefore, they are supplementary angles, meaning that they add up to 180 degrees.
Since EFC and EFD are adjacent and form a straight line, they fulfill the criteria for a linear pair.
Thus, the angle relationship that best describes angles EFC and EFD is:
Linear pair.