In answering these questions, it is assumed that x is always the input and y is always the output when deciding whether we have a function or not.
So:
(1) Function, because for every x there is just one y.
(2) Not a function, because x=4 maps to two different y.
(3) Function, because for every x there is just one y, even though it’s the same value.
(4) Function for the same reason as in (3).
(5) Function, because each x goes to one y.
(6) Not a function of x, because x goes to two different values of y. (But it is a function of y because every y goes to one value of x.)