The correct additional information is that EF = GF.
To prove that DEFG is an isosceles trapezoid, you need to show that either DE || FG or DE = FG.
Looking at the options:
- DE || DG: This is not sufficient information because it only tells us that DE and DG are parallel, but it doesn't provide any information about FG.
- DE = DG: This is not sufficient either because it only tells us that DE and DG have equal lengths, but it doesn't provide information about the parallelism with FG.
- EF = DG: This is not sufficient as it only states that EF and DG have equal lengths, but it doesn't give information about DE and FG.
- EF = GF: This is the option that would prove DEFG is an isosceles trapezoid. If EF is equal to GF, it means that the non-parallel sides DE and FG are equal in length, making DEFG an isosceles trapezoid.
Therefore, the correct additional information is that EF = GF.