Function:
A function is a relation in which every input x has a different output value y.
If there is more than 1 outputs for the same input then it doesn't represent a function.
Let us analyze each of the given options.
Option A:
{(2, -1), (4, -2), (6, -3), (8, -4)}
As you can see, each input x has a different output value y.
Therefore, it is a function
Option B:
{(0, 0), (1, 1), (1, 0), (2, 1)}
As you can see, the input (1) has two different outputs (1 and 0)
Therefore, it is not a function.
Option C:
{(3, 3), (3,4),(4,3),(4,4)}