If, on average, more umbrella sales occur on days with more rainfall, then the strongest statement we can make is:

The strongest statement we can make in this scenario is that rainfall and umbrella sales are associated with one another. This conclusion is based on the observation of a correlation between the two variables. When analyzing data where one variable shows a consistent pattern with another, such as increased umbrella sales coinciding with heightened rainfall, it suggests a relationship.

However, it's crucial to understand that correlation does not imply causation. Although umbrella sales increase with more rainfall, we cannot definitively conclude that changes in rainfall directly cause changes in umbrella sales or vice versa, which is why choices that imply a direct cause-and-effect relationship are not appropriate here. Additionally, while seeking more data can be beneficial for a more robust analysis, the existing data already indicates a noticeable association between rainfall and umbrella sales, making the statement regarding their association the strongest and most accurate conclusion that can be drawn at this stage.