Which of the following relations are functions?
a. {(ad)}
b. {(c,,,b)}
c. {(ba)}
d. {(ae)}

{(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.