Context: Right triangle, legs $8$ and $b$, hypotenuse $20$.

Answer: Pythagorean Theorem: $a^2 + b^2 = c^2 \Rightarrow 8^2 + b^2 = 20^2$

Why C: Correct application of the theorem with legs squared summing to hypotenuse squared.

Traps: A ($8b=20$) uses product, not squares. B ($8+b=20$) linear sum, not squares. D ($8^2-b^2=20^2$) subtracts instead of adds.