Which relation is a function?

A. (3,4), (3,3), (1,2), (1,5)

B. (3,4), (2,4), (5,1), (3,5)

C. (1,3), (2,6), (3,9), (4,12)

D. (1,4), (1,3), (1,2), (1,1)

C

Step-by-step explanation:

Look for the case in which every input is unique (no repetitions).

A does not qualify, as 3 appears as an input twice.

B does not qualify, for the same reason

C qualifies (is a function), because all the inputs {1, 2, 3, 4} are unique.

D does not qualify (see A, above).