Answer:
a. Alternate exterior angles.
b. <A and <B are congruent. This diagram involves a line intersecting two parallel lines, forming the congruent angles <A and <B on opposite sides of the transversal.
c. They are alternate exterior angles like <A and <B, but because it is not guaranteed that the transversal is intersecting parallel lines in this case, we cannot prove that <C and <D are congruent alternate exterior angles.