Which one would it be?

Answer:
B
Step-by-step explanation:
Because there are 2 different y values for the x value of 5. All x values can only have 1 y value.
For B, there are y points on the x value of 5, so this fails the vertical line test, and is therefore not a function
I believe the correct answer is D. This is because when you match up the function, x is always similar to y.