The area of trapezoid ABCD is
... Area = (1/2)·(BC +AD)·(BF)
We can fill in the given information and solve for the remaining variable.
... 30 = (1/2)(6 + AD)(4) = 12 + 2·AD
... 18 = 2·AD . . . . . subtract 12
... 9 = AD . . . . . . . divide by 2
Since AB || CE, the lengths BC and AE are equal; both are 6. So, we have
... AE + ED = AD
... ED = AD - AE = 9 - 6
... ED = 3 . . . . . corresponds to selection (A)