A function is an equation or graph where each x value only has one y value. In the first image you can use the vertical line test. Take your pencil and lay it on the paper vertically. Move it across the graph. Basically if an x value has multiple y value it is not a function. The first image is not a function because all x values past zero have multiple y values for each x value. In the second image it is not A because in A the x value of 2 has a y value of 1 and 8. It is not B because the 5 has a y value of 3 and 7. It’s is not D because of the 5 again. It is C because all x values are unique.
Hope that helps