The association described in the scenario is that the Dudley family will likely spend more money on diapers than their neighbor, who has one baby. This indicates a positive association, as having more babies (twins, in this case) generally leads to increased spending on diapers compared to having a single baby.
However, it's important to note that this relationship is not a causation but rather a correlation; the presence of more babies (twins) correlates with more spending on diapers, but it does not cause the increase in spending.
Therefore, the correct response is:
D positive; correlation