A function maps one input to one output. If an input maps to more than one output, the relation is not a function.
In order to make the mapping be not a function, you must show at least one member of set A being mapped to more than one member of set B. (It does not matter which input you choose, or which outputs you choose.) More than one member of set A may have multiple mappings.
In the attached diagram, I have mapped 1 to both a and c to make this be not a function.