Step 1: Understand the behavior of the function.
The function \( f(x) = \frac{1}{a^x} \) is an exponential function with base \( a \), where \( a>0 \). Since the base \( a \) is greater than 1, this function is decreasing as \( x \) increases. The rate of decrease becomes slower as \( x \) increases.
Step 2: Conclusion.
Thus, the function is decreasing, corresponding to option (A).