Which graph is a one-to-one function

The graph which is a one-to-one function is:

                      Graph B

Function--

The graph of a function must always pass the vertical line test i.e. any line passing through the domain and parallel to the y-axis must pass through the graph exactly once.

and also for a function to be one-one the graph of the function must pass horizontal line test i.e. any line passing through the co-domain and parallel to the x-axis must intersect the graph at most once.

In the given graphs :

Graph A is not a function, since it does not pass the horizontal line test and hence we won't talk about being a one-one function.

Graph B is a function as well is one-one since it passes horizontal as well as  vertical line  test.

Graph C is a function but is not one-one as it fails the horizontal line test.

               Hence, the correct answer is:

                          Graph B

i do not know this sorry : (