The answer is D. 2√5 cm.
See attached file.
By symmetry, the red vertical line measures 4 cm. On the other hand, the red horizontal line measures 5 - 3 = 2 cm.
Notice that the line segments that measure 2 cm, 4 cm, and [tex]x[/tex] form a right triangle wherein [tex]x[/tex] is the hypotenuse of the right triangle. So by Pythagorean Theorem, we find that
[tex]2^2+4^2=x^2[/tex]
Solving for [tex]x[/tex] yields [tex]x=\sqrt{20}=2\sqrt{5}[/tex].
