Question

Which relation is a function?

Answer 1

The relation shown in the 2nd diagram is a function.  The reason for this is that one and only one y-value is associated with each x-value.  

Note that this is not true of the first diagram.  If x=0, there are two separate y vaues associated: -1 and 2.  This clearly states that the first diagram does not represent a function.

Answer 2

Of the provided graphs, the second would be the correct answer.

Functions occur when the input only has one possible output (though the output can be recieved through multiple inputs)