Step 1: Understand the Pythagorean Theorem.
The Pythagorean theorem states that in a right triangle, the square of the length of the hypotenuse ($ c $) is equal to the sum of the squares of the lengths of the other two sides ($ a $ and $ b $). Mathematically, this is expressed as:
$$
c^2 = a^2 + b^2.
$$
Step 2: Analyze Each Option.
Let's evaluate each option based on the Pythagorean theorem:
1. Product:
Incorrect, as the theorem does not involve multiplying the squares of the sides.
2. Sum:
Correct, as the theorem explicitly states that the square of the hypotenuse is the sum of the squares of the other two sides.
3. Difference:
Incorrect, as the theorem does not involve subtracting the squares of the sides.
4. Ratio:
Incorrect, as the theorem does not involve dividing the squares of the sides.
Step 3: Final Answer.
$$
(2) \mathbf{\text{Sum}}
$$