Answer
Not a function.
Work Step by Step
A function is a correspondence between a first set, called the domain,
and a second set, called the range,
such that EACH member of the domain corresponds to EXACTLY ONE member of the range.
---
Keywords are emphasized.
We see from the graphical definition of the correspondence that
2 from the domain has more than one corresponding value in the range (a and b).
So, the "exactly one" clause has been violated.
Not a function.
Note: The same clause is violated for 4 from the domain.
