21. Identify the congruence criteria for triangles CFD and EFD based on the markings. Provide a step-by-step explanation.

Step-by-step explanation:

• Step 1: Analyze the markings. In triangle CFD, we observe that side CF is marked with a single dash, indicating it is equal in length to side EF. Side FD is common to both triangles CFD and EFD. Angle C and angle E are marked with a single arc, indicating they are equal.
• Step 2: Identify the congruence criterion. We have two sides and an angle. However, the angle is not included between the two sides (it's opposite to one of the sides).
• Step 3: Consider other congruence postulates. Let's re-examine the markings. If CF = EF (Side), FD = FD (Side), and $$\angle C = \angle E$$ (Angle), this does not directly match Side-Side-Angle (SSA) which is not a valid congruence criterion in Euclidean geometry for all cases. Let's look closer at the image. The markings on sides CD and ED are double dashes, implying CD = ED.
• Step 4: Re-evaluate with all markings. Now we have: CF = EF (Side), CD = ED (Side), and FD = FD (Side).
• Step 5: Apply the SSS congruence criterion. Since all three sides of triangle CFD are equal to the corresponding three sides of triangle EFD (CF=EF, CD=ED, FD=FD), the triangles are congruent by the Side-Side-Side (SSS) congruence postulate.

Answer: Triangles CFD and EFD are congruent by the SSS (Side-Side-Side) congruence criterion.