SOLUTION
The diagram below shows a vertical line test
The picture above shows the vertical line test
For this test, if a vertical line is drawn across the graph at any point, if it cuts the graph once, then it is a function.
If the line cuts the graph more than once, then it is not a function
In the graph a. the two lines we made cuts the graph only once as see above.
But in the graph b. the line we made cuts the graph at two points.
Hence
graph a. is a function and
graph b. is not a function