A function is a relation in which the members of the domain (x-values) DO NOT repeat. So, for every x-value there is only one y-value that corresponds to it. y-values can be repeated.
A function, f(x), cannot have repeating x-values. Option D has the number 5 repeated twice, {1, 5, 5, 7}, in the x-column, therefore it cannot be a function of x.
