Which type of datasets can be added to a root dataset to narrow down the search?

The correct choice is the child dataset. Child datasets are specifically designed to refine or narrow down the scope of a search that is applied to a parent dataset. When you add a child dataset to a root (or parent) dataset, you create a hierarchical relationship where the child dataset inherits properties from the parent, but can also introduce additional criteria or filters. This allows users to perform more targeted searches by focusing on specific subsets of data under the broader category defined by the parent dataset.

Child datasets can be used to implement more detailed analyses, such as focusing on transactions, specific user activities, or particular time frames, making them essential for drilling down into data that falls under a larger dataset umbrella.