Answer:
Line ED
Explanation:
First, looking at the diagram ED is the only line plane ADE and CED have in common.
Plane ADC touches both planes, but the intersection of 2 planes cannot be another plane.
Plane AEC is actually not a plane, I don't think. Could be wrong but regardless the same explanation as before.
Line AC does not touch both planes ADE and CED.
Line ED borders both AED and CED. It is inbetween the two planes, and is the only line in common between both planes.