The answer is B. The angle GAB is opposite to EAD. Opposite angles are always equal.
If you want to make sure even further, we know that angle DAC and EAD are complementary, as the sums of their angle measures are 90 degrees. Therefore, angle DAC is 40 degrees. Angle GAF is opposite to this angle, meaning it is also 40 degrees. Now we can see that the angle measure is correct fro GAB because it is complementary to GAF. 40+50=90.
Therefore, the answer is B.