Answer:
B. X
Explanation:
the essential rule for a function is that each value of x must have exactly one associated y value.
in other words, if we "feed" the x- value to the function it must be clear what y-value we get as result.
in W 6 has 2 associated y values. no function.
in X all is OK. yes, a function.
in Y again, 6 has 2 associated y values. no function.
in Z 7 has 2 associated y values. no function.