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 means that as one variable (rainfall) changes, there is a corresponding change in the other variable (umbrella sales). The observation that more umbrella sales occur on days with more rainfall indicates a correlation between the two factors, reflecting a consistent pattern in the data.

This relationship does not imply causation; just because umbrella sales increase when it rains, we cannot definitively conclude that rainfall causes umbrella sales to increase, or vice versa. The other statements suggest a direct causal relationship, which is not supported by the data given. The acknowledgment of the association allows for further exploration into potential underlying causes without making unwarranted assumptions about one variable affecting the other directly. Hence, recognizing that rainfall and umbrella sales are associated captures the essence of the observed data accurately.