SOLUTION
The horizontal line test for one-to-one function explains that a function is one-to-one if the horizontal line cuts the function at only one point.
Hence, considering the graph naming the vertically from left to right
1. Graph 1 is one-to-one since the horizontal line cut the graph once (YES)
2. Graph 2 is one-to-one since the horizontal line cut one of the graphs at a point (YES)
3. Graph 3 is not one-to-one since the horizontal line cut the graph more than once (N0)
4. Graph 4 is not One=to=one since the Horizontal line did not cut the graph at any point (N0)
5. Graph 5 is one-to-one since the horizontal line cut one of the graphs at a point (YES)
6. Graph 6 is one-to-one since the horizontal line cut one of the graphs at a point (YES)