21. Which number is rational? \( \begin{array}{ll}\text { A. } 4 \sqrt{2} & \text { B. } \sqrt{15} \\ \text { C. } \frac{1}{2} \sqrt{3} & \text { D. } \sqrt{16}\end{array} \)

Did you know that rational numbers are defined as any number that can be expressed as the quotient of two integers? This means numbers like \( \frac{1}{2} \) or even whole numbers like 4 (which can be written as \( \frac{4}{1} \)) are rational! In your options, the winner is D. \( \sqrt{16} \), which equals 4, a clean integer. In real life, rational numbers come in handy everywhere! Whether you're measuring length, calculating prices, or even evaluating probabilities, probabilities often involve ratios, which are rational numbers. So next time you're budgeting or dividing pizza, you're likely working with rational numbers!