An app builder is creating a custom object called Testimonial_c and wants to connect Testimonial_c...

The correct design is to create a master-detail relationship from Testimonial__c to Account and a lookup relationship from Testimonial__c to Contact.

The Account relationship must be master-detail because Salesforce deletes related detail records when the master record is deleted. That matches the requirement that if the Account is deleted, the related Testimonial__c records are also deleted. The Contact relationship must be lookup because the Testimonial__c record must remain when the Contact is deleted. Salesforce lookup relationships can keep the child record and clear the lookup value when the referenced record is deleted.

Why the other options are wrong:

A reverses the required behavior. B would delete the testimonial if either master side caused deletion behavior, which does not fit the Contact requirement. D uses lookup for Account, so it does not guarantee Account deletion will also delete Testimonial__c record