Which of the following relations are functions?

A. { (a, b), (b, a), (c, c), (e, d) }

B. { (c, e), (c, d), (c, b) }

C. { (b, b), (c, d), (d, c), (c, a) }

D. { (a, b), (b, c), (c, d), (d, e) }

{ (c, e), (c, d), (c, b) } is NOT a function since the input c has multiple outputs (e, d, b). So choice B is out

{ (b, b), (c, d), (d, c), (c, a) } is NOT a function either. The input 'c' corresponds to the output 'd' and 'a' at the same time. So choice C is out too

Choices A and D are the answer. They are functions since any given input corresponds to exactly one output.