Which of the following tools would you use to combine datasets in Alteryx?

The Join tool is designed specifically for combining datasets based on a common field or key from each dataset. It allows you to match records from two separate datasets effectively and brings the relevant data into a single output based on that relationship. This capability is crucial when you want to create a new dataset that contains only the records where there's a match, and it provides options to manage how unmatched records are handled as well.

On the other hand, the Union tool is used to stack datasets on top of one another rather than merging them based on a common key. The Blend tool, while it involves combining data, is more focused on visual analysis and is not a standard tool for merging datasets in the same way as the Join tool. The Merge tool typically refers to combining multiple streams in certain contexts but does not function the same way as the Join tool in dataset merging.

Given these definitions and uses, the Join tool is the most effective and specialized option for combining datasets based on shared identifiers.