Which provider should be extended to add a custom column in the grid?

The correct choice is the ColumnsProvider, as it is specifically designed to manage and extend the columns displayed in data grids within Sitefinity. By extending the ColumnsProvider, you gain the ability to add custom columns to an existing grid, enabling you to tailor the grid's appearance and functionality to meet specific needs or display additional data.

This is essential for effectively presenting information that is not covered by the standard configuration of Sitefinity's grid system. By utilizing the ColumnsProvider, you can define the structure of your grid, including custom logic on how the data should be displayed in these new columns.

In contrast, the other providers mentioned serve different purposes. The ItemsProvider primarily deals with managing and providing access to the underlying items or data entries that populate the grid. The DataProvider is focused on the general data handling aspects, such as querying and manipulating data, but does not specialize in customizing the grid's column structure. The WidgetProvider, meanwhile, is responsible for managing the overall widget definitions and their associated behaviors but does not directly influence the columns within a grid.