The correct reason that justifies statement 6, which asserts that \((EF)^2 = (AC)^2 + (CB)^2\), is:
Pythagorean Theorem
This is because statement 6 applies the Pythagorean Theorem to the right triangle DEF, where the sides \(FD\) and \(DE\) correspond to sides \(AC\) and \(CB\) of triangle ABC, respectively, and \(EF\) corresponds to the hypotenuse \(AB\).