Let U $=\{$a$,$ b$,$ c$,1,2\}.$

Let A $=\{1,$ c$,2\},$ B $=\{$b$,$ c$,1\}$ and C $=\{$a$,$ c$,1,2\}.$

Which one of the following relations from A to C is functional?

Select one:

a$.$

$\{($a$,$ a$),(2,2),($c$,$ c$)\}$

b$.$

$\{($c$,$ a$),($c$,$ c$),($c$,1),($c$,2)\}$

c$.$

$\{($c$,1),(2,2)\}$

d$.$

$\{($c$,$ a$),(2,$ c$),(1,1),($c$,2)\}$