Question 309353
A function f from A to B is called one-to-one (or 1-1) if whenever
f (a) = f (b) then a = b.   No element of B is the image of more than one element in A.

In other words, a one-to-one function CANNOT have repeated values for y.

Looking at your choices, the only one that does not repeat values for y is choice 2 or (1,3)(2,4)(4,1)(0,2).

Do you see that the values for y are 3, 4, 1 and 2?  None of them repeat and so this is why it is one-to-one.