Context: Right triangle, hypotenuse $21$, leg $6$. Find other leg $a$.

Answer: A: $\sqrt{21^2 - 6^2}$

Why: $a^2 + 6^2 = 21^2 \Rightarrow a = \sqrt{21^2 - 6^2}$.

Traps: B ($21^2 - 6^2$) gives $a^2$, not $a$. C ($\sqrt{21-6}$) subtracts before squaring. D ($21-6=15$) ignores the theorem.