Hello!
In a function, each input has only one output. In A, three has two outputs, 4 and 5, so A is not a function.
In B, you can use something called the vertical line test to see if each x value has one y value as an output. You move an imaginary vertical line across the graph, and if it intersects with two points it is not a function. If we do this on our graph, it will not intersect two points. Therefore, B is a function.
In C, we can see that each input has one output, or there are all different inputs, so C is a function.
For D we can use that vertical line test again. It intersects both the points (-1,1) and (-1,6) so D is not a function
Our final answers are B and C.
I hope this helps!