Answer: d. alternate interior angles
Note how the two angles are inside, or interior, of the lines L1 and L2. For me, I tend to think of such lines like train tracks. So that takes care of the "interior" portion.
As for the "alternate" portion, we can see that the angles are on either side of the transversal line T.
So that's why angles 3 and 6 are alternate interior angles.
If L1 and L2 are parallel, then angle 3 = angle 6.
