The expression \( \sqrt[3]{21} \) can be rewritten with a rational exponent. The square root is represented using a rational exponent of \( \frac{1}{2} \) and the cube root using \( \frac{1}{3} \).
To rewrite \( \sqrt[3]{21} \) using a rational exponent, we express it as:
\[ 21^{\frac{1}{3}} \]
This corresponds to the response:
\( 21^{\frac{1}{3}} \)
If you're looking at the options you've provided, \( 21^{\frac{1}{3}} \) would match with the format \( 21 \text{ superscript } \frac{1}{3} \).