Congruence Theorems
SSS congruence theorem states if all three sides of two triangles are equal, then the triangles are congruent.
HL (Hypotenuse-Leg) theorem states if the hypotenuse and one leg of one right triangle are congruent to the hipotenuse and leg of another right triangle, then they are congruent.
Please note the triangles CEF and DEF on the figure have the same tick marks on their hypotenuse and one leg and they are right triangles. This ensures they are congruent by the HL theorem.
But since both triangles share a common side (EF), then all the sides are congruent, and the SSS theorem also applies, thus if you have to circle one, do it with HL, just for being the most obvious, but both theorems apply here.