If two planes intersect, they intersect in a line. This line consists of all common points of both planes.
The plane ABFE contains points A, B, F and E, because these points are in the name of the plane.
Points B, F, C and D lie on the plane BFCD.
As you can see points B and F are common points of planes ABFE and BFCD, this means that the line of intersection of these two planes is BF.
Answer: BF.