Final answer:
The HL (Hypotenuse-Leg) theorem states that if the hypotenuse and a leg of one right triangle are congruent to the hypotenuse and corresponding leg of another right triangle, then the two triangles are congruent.
Step-by-step explanation:
The HL (Hypotenuse-Leg) theorem states that if the hypotenuse and a leg of one right triangle are congruent to the hypotenuse and corresponding leg of another right triangle, then the two triangles are congruent.
In order to prove triangles are congruent using the HL theorem, you need to know the side lengths and a right angle.
Therefore, the correct option is C. Side lengths and a right angle.