In $8^9,9$ is the ______.

The correct option is A. power

Any large number can be written in exponential form on the basis of their factors. Let's take a number $64$. The prime factorization of $64$ is $2\times 2\times 2\times 2\times 2\times 2$. This can be written as factor as base and number of factors come in prime factorization as a superscript or power. So, $64=2^6$.

In $8^9$, $8$ is the base and $9$ is the power.

Hence, option A is correct.