Recall that when two angles add to give 90 degrees, they are called "complementary angles" This is the case here where you are adding an unknown angle to one that measures 51 degrees to obtain a right angle (90 degrees)
The unknown angle (?) is called the "complementary angle to the 51 degree one.
Please select the third option :
C. Complementary.
Do you remember how it looks an angle that measures 90 degrees? It looks like the angle formed by a square edge, or by two lines that are perpendicular to each other.
When you have two angles one in contact (adjacent) to the other and the two together complete a right angle , those two angles are called complementary angles. There is nothing profound to derive from that, but that it is simply a definition.