The biggest rule for functions in a table is that none of the numbers in the X column are repeating. But for the Y column, it is okay to have repeats.
When you look at the answer choices, you could eliminate B, C, and D because of repeats in the x column.
A IS A FUNCTION because all the numbers in the x column are different
B is NOT A FUNCTION because in the x column, there are 2 3's
C is NOT A FUNCTION because in the x column, there are 2 -5's and 2 -10's
D is NOT A FUNCTION because there are 2 20's
Your answer would be A.