Final answer:
The Group by attribute is defined in the c)Grouping Options phase of the Report designer.
Step-by-step explanation:
The phase of the Report designer that lets you define the Group by attribute is the C) Grouping Options.
In the grouping options phase, you can organize data into meaningful categories to aid in analyzing the report data. This step is crucial because it enables the report to display data summed up or categorized based on a specific field or criterion. For example, if you are creating a sales report, you might want to group data by region or by product type so it's easier to see sales patterns and trends related to these categories.