The answer is B!
In a function, the x input cannot be repeated.
In A, you see that 4 has an arrow pointing to 2 y inputs. That means that 4 is repeated and cannot be a function.
In C, 2 and 5 are both repeated x inputs, so they cannot be functions either.
In D, 0 and -1 both have arrows pointing to 2 y inputs. As stated before, if an arrow points to more than one number, it cannot be a function.
In B, all the arrows point to only ONE y input and none of the x inputs are repeated. Thus, the correct answer is B!