Remember the fact that an exponent shows how many times a base is being multiplied by itself. For example:

In the above example, you can see that there are four 5's being multiplied together. Thus, we can simplify the expression to an exponent of 4 with a base of 5, as shown.
In this specific problem, we see that three 9's are being multiplied together. Thus, the exponent of our term would be 3 and the base would be 9. This is represented as
.
The answer is the first choice (assuming it is 9^3).