A. All cohorts in your project can be used as a base cohort for index reporting.
You can choose 'Everyone' or select a specific cohort to be your base cohort.
Example: If you have global audiences you might want to run an index report by GEO location, ie. comparing your UK and US visitors. To do this: Go to Audience Insights, click the Index and Overlap tab, run an audience report on your UK Visitors cohort, and filter your base cohort to select the US Visitors cohort.