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.