Based on the description provided, it seems that the scatter plot of daily stock market returns shows a generally horizontal line around 0% return, with most data points lying below the line and in the negative range. This suggests that there might not be a clear functional relationship between the stock's return and time.

Given that the scatter plot does not indicate a clear pattern or trend that can be adequately described by a specific function, the best choice would be:

D. No function best explains the relationship between the stock's return and time.

This option acknowledges that the data doesn't conform to a specific mathematical function and that the relationship between the stock's return and time may be more complex or unpredictable.