pairs of angles are always congruent to each other.           a. supplementary   b. complementary   c. adjacent   d. vertical

The correct answer would be D. Vertical angles, since angles that are vertical from one another are always equivalent. As according to a theorem, whenever you have two vertical angles, their measurement of angles will always be the same. Choice A does not make sense since while it is possible for two supplementary angles to be the same, sometimes there might be two angles such as 100 and 80 which make a supplementary angle even though they aren't congruent. The same also applies to complementary angles since there could be an 80 degree angle and a 10 degree angle. The answer would also not be choice C as not all adjacent angles are equivalent to each other. For example, there could be an angle that shares the same side as another but they have different angle measures such as a 23 degree angle and a 40 degree angle who happen to be adjacent.