Answer:
The radical expression which represents a to the four ninths power is:
Ninth root of a to the fourth power.
Explanation:
We are asked to find the radical expression for the word phrase:
a to the four ninths power.
i.e. mathematically it could be written as:

Now, we know that:
![a^{(m)/(n)}=(a^m)^{(1)/(n)}=\sqrt[n]{a^m}](https://img.qammunity.org/2020/formulas/mathematics/middle-school/7hgcat6nt2crb4ey09gyd6c8r3hf99siqp.png)
Here we have:

Hence, the expression cold be written as:
![a^{(4)/(9)}=\sqrt[9]{a^4}](https://img.qammunity.org/2020/formulas/mathematics/middle-school/f01v3ra22grr8ysk65akywgkwd3mg2tcc5.png)