Answer:
c). [tex]2\sqrt{2}[/tex]
Step-by-step explanation:
It's a right triangle with equal sides (DE == EF).
We know that
[tex]4^2 = DE^2 + EF^2 = 2\cdot DE^2\\16 = 2\cdot DE^2\\8 = DE^2\\4\cdot2 = DE^2\\2\sqrt{2} = DE\textrm{; because }-2\sqrt{2} = DE\textrm{ doesn't make sense in geometry}[/tex]
