Do One-to-One functions have to pass both, vertical and horizontal line test or just one? Explain :D?

Answer 1

Yes. In order to be a function it needs to pass the vert line test. In order to be one-to-one, the function needs to pass the horz line test. (So if you already know that the graph is a function, you only need to do the horz line test.)

Answer 2

they must pass both
reread the definition of one to one
each value of x yields only one unique value for y,
and each y value is obtained from a single unique value of some x