To convert the expression \( 3\sqrt{216} \) into an expression with a rational exponent, we first express the cube root as an exponent. The cube root can be written as raising to the power of \( \frac{1}{3} \). Therefore, we can rewrite the expression as:
\[ 3\sqrt{216} = 216^{\frac{1}{3}} \]
Since this is equivalent to \( 216^{1/3} \), we see that the rational exponent is:
\[ \frac{1}{3} \]
Thus, the correct response is:
\(\frac{1}{3}\) (Start Fraction 1 over 3 End Fraction).