Answer:
The bottom left graph is a function
Explanation:
I was pretty confused at first with this, too. To find if a graph is a function, however, just preform the vertical line test! Basically, draw a vertical line on each graph. At any point does it go through two or more points? Well, its not a function. We can see a vertical line would completly overlap the first graph, it would go through the half circle graph twice, and the circle? Well, that ones rather obvious...
With this test, we can see the only graph that passes the vertical line test is the bottom left graph (or the weird squiggly one)!
Hope this helps for future function solving!