In the process of creating a subreport, what must be linked to the main report?

When creating a subreport, it is essential to link it to the main report using a common field, such as "ID." This linking establishes a relationship between the two reports, ensuring that the subreport can fetch and display relevant data based on the context provided by the main report.

By utilizing a common field, you allow the subreport to filter or organize its data according to the records being displayed in the main report. For instance, if the main report lists customers, the subreport may show orders for each specific customer. The common field acts as the connection point that guides how the data from the subreport corresponds to the records in the main report, ensuring coherence and relevance.

The other options do not provide the necessary link between the main and subreport. For instance, simply linking by the name of the report does not establish a connection to the data. Similarly, an external database connection is not required if the data is already available within the context of the main report. Lastly, focusing solely on layout settings disregards the data linkage, which is crucial for the functionality of subreports.