A. Matrix

To accomplish the given requirements, a Matrix report format should be used. A Matrix report displays data in a grid, with groupings both vertically and horizontally. The grouping of data in a matrix report can be changed dynamically by the viewer using the drop-down menus available in the report. This means that the sales users can group their open Opportunities by Stage or by Source on the same report by selecting the desired grouping in the drop-down menu.

In a Matrix report, the left-hand column displays the vertical grouping, while the top row displays the horizontal grouping. The summary data for each intersection of the vertical and horizontal grouping is displayed in the body of the report.

Option B (Joined) and D (Tabular) report formats do not allow grouping of data both vertically and horizontally, so they are not suitable for this scenario. Option C (Summary) report format only allows grouping of data in one direction, so it is not suitable for this scenario.