For any of these, ask yourself this question: "is it possible to draw a single vertical line and have this vertical line pass through more than one point on the curve?"
The answer is "no" for all of these questions. It is impossible to draw a single vertical line through more than one point on the curve. So each of these graphs represents a function. One input (x) goes in, leading to exactly one output (y) coming out. This is assuming that the x value is in the domain of course.
In short, they are all functions because they pass the vertical line test.