Which of the following statements proves that J K L is a right triangle?

a. the sum of the lengths of two of the sides is equal to the length of the third side.
b. the sum of the lengths of two of the sides is greater than the length of the third side.
c. the sum of the squares of the lengths of two of the sides is equal to the length of the third side.
d. the sum of the squares of the lengths of two of the sides is equal to the square of the length of the third side.