Which of them are functions and not functions?

Hello!

In a function each input has ONE output. Therefore, to solve you must see if each input (numbers in the left oval) point to ONE output (has ONE arrow pointing to ONE number in the right oval).

In 13,  has ZERO outputs, and it must have at least one to be a function, therefore 13 is not a function.

In 14, each number has one output, so 14 is a function.

In 15,  points to both 2 and 3, meaning it has two outputs, therefore 15 is not a function.

In 16,  points to both 2 and 3, so 16 is not a function. Note that 4 has two inputs. This is acceptable because 4 is the only number each of the numbers go into.
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Final Answers

13. Not a Function
14. Function
15. Not a Function
16. Not a Function

I hope this helps!