D. (2, 2, 4) is not a pythagorean triple.
This is because:
sqrt(2^2 + 2^2)
= sqrt(4 + 4)
= sqrt(8)
=/= 4
For a set of numbers (a, b, c), where c is the largest number out of the three, to be a pythagorean triple, sqrt(a^2 + b^2) must be equal to c.
However, for D., this is clearly not the case.