You want to use the classic workflow in your business process flow. 
Please select two cases when you can trigger an on-demand classic workflow in a business process flow.

Answer options:

A.At the end of the first step
B.On a stage transition
C.As a new condition
D.On a change of business process flow status
E.On a schedule

Correct Answers: B and D 
Power Automate Business process flow can include on-demand classic workflow. The classic on-demand workflow can be triggered by a business process flow stage transition or a business process flow status change. 
In a Business process designer, you can add a workflow to the stage (Number 1). You need to provide the stage transition trigger. There are two options: Stage Entry and Stage Exit (Number 2). And select the workflow that you want to run (Number 3). Your workflow must be active, on-demand, and created for the same table that you are using in the stage. 

A stage transition triggers do not work for the first and the last stages in a business process flow. If you want to use these stages, you need to create a Global workflow (Number 1). 
The Global workflow is triggered by the changes of the business process (Number 2). There are four process changes: Process applied — when business process flow is started
Process abandoned — when a user abandons a business process flow, and it is archived
Process reactivated — when a user restarts an abandoned business process flow
Process completed — when a user clicks to finish the final stage and ends the business process flow.
And finally, you need to provide a workflow to run as a Global workflow (Number 3). 

All other options are incorrect. 
For more information about Business process flow and classic on-demand workflow integration, please visit the below URLs: 

https://docs.microsoft.com/en-us/power-automate/bpf-add-on-demand-workflow
https://docs.microsoft.com/en-us/power-automate/business-process-flows-overview#business-process-flows-that-call-a-workflow
https://docs.microsoft.com/en-us/power-automate/flow-types