Final answer:
Without a specific graph or function, we cannot determine which interval indicates where function f is decreasing among options A, B, or C.
Step-by-step explanation:
To determine which interval indicates where the function f is decreasing, we need to consider the behavior of function lines on a graph. If line A is increasing and steeper than line B, and line B is decreasing, then we are focused on where line B represents the function f. However, without a specific graph or function provided in the details here, we cannot definitively choose one of the given options A, B, or C as the interval where the function is decreasing. It is essential to analyze the graph or the mathematical expression of the function to determine its increasing or decreasing intervals accurately.