Recall the pencil line test, which is where you go across the x axis (the graph horizontally) and see if the pencil touches any two points at the same time. If it does, then the graph is not a function.
Point being: since question a has a bunch of y values on top of one x value, the pencil would inevitably touch multiple at the same time, hence why it is easy to tell that it is not a function! Hope this helps :)
