I consider this a bad question.

Ultimately, the question states:

They have a custom Salesforce application to manage customer support cases, but only certain cases need to be visible to external partners - not all cases.
Also, my understanding is that the support cases aren't created by/for the external partners.

This would suggest Apex managed sharing.

But the fact that they include sharing sets, and how they seem to hint that this is a community portal with community users, makes this a bad question.

No this is incorrect , the answer is D
A sharing set grants portal users access to a record associated with an account or contact that matches the user’s account or contact
( Case object is included on this feature , so yeah )
Reference : https://help.salesforce.com/s/articleView?id=sf.fsc_admin_set_up_sharing_sets.htm&type=5