There is a vertical line test you should use it to test if the figure is function or not
Draw a vertical line through the figure many times
If the line intersects the figure in more than 1 point, then the figure is not a function
The vertical line should intersect the graph of the function at one point only
Figure 1 is a triangle
If you draw a vertical line at any part of it
It will intersect in 2 points
So figure 1 not a function
Figure 2 is a parabola
Draw a vertical line in many parts of it
It will intersect it in only 1 point
So figure two is a function
Figure 3 represents the absolute equation
Draw a vertical line to test it
It will intersect in more than 1 point
So figure 3 is not a function
Figure 4 have different points
Draw a vertical line passing these points
Is this line passing throw more than one point?
No, so figure 4 is a function
Figure 5 represents two horizontal segment
If we draw a vertical line it will cut each line in one point but cut the graph in all in two points
So figure 5 is not a function
In figure 6 there are some points
two of them