Which of the following relations is a function?

Question

A. {(5, 2), (3, 2), (4, 2), (5, 1)}
B. {(1, 2), (1, 3), (2, 1), (3, 4)}
C. {(1, 3), (2, 6), (3, 8), (1, 7)}
D. {(1, 4), (8, 9), (10, 11), (12, 12)}

I say c

Answer 1

If for every x, there is one and only one value of y, we have a function

in your c)
I see 1 --->3
and 1 ----> 7
oops, contradiction!
same is true for a and b
mmmmhhh, look carefully at D

Answer 2

in b, x=1, y=2 or 3 not a function
in c, x=1, y=3 or 7, not a function
In a, x=5, y can be two values. not a function
in d. each x has a related unique y.