Only table D is a function. All of the others fail the "vertical line test." The definition of a function is for each input there is only one unique output. So for example, any given input value of x, there can only be one unique output value for y.
Michael is 8 and Murphy is one year younger, so that makes him 7. The twins are two years younger than Murphy, so that makes them 5 years old. Isabelle is 5.