Answer B is correct

If we have to select one only then asnwer should be B: Roll-up summary fields exist on the master object.

B is correct, but D is also correct :
Data integrity: Changing a Master-Detail relationship to a Lookup can result in orphaned child records if the parent record is deleted or changed. Salesforce prevents this by enforcing data integrity rules that require you to ensure that there are no orphaned records before making the change.

answer is B