Answer:
A
Step-by-step explanation:
A function is in one to one correspondence, that is for any value in the input there is only one unique value in the output
set A satisfies this and is therefore a function.
set B has (2, 1) and (2,4) ← not a function
set C has (3, 4) and (3, 5) and (4, 2), (4, 5) ← not a function
set D has (5, 2) and (5, 3) and (4, 2), (4, 5) ← not a function
