D, because you can't create one skinny table for both account and contact. Remember the documentation says 'Skinny tables can not refer fields from another object.' Skinny table only avoids a join to fetch standard and custom fields for one object. so contact skinny table can include related account Id but not any other fields from account object. It is not possible to create one skinny table for both account and contact object fields

C. Create a skinny table to represent account and contact objects.

Skinny tables can help speed up data retrieval times for reports and list views. They contain frequently used fields and records, allowing Salesforce to bypass the standard query optimizer. This can be particularly useful when dealing with large data volumes, as in this case with 2,500,000 accounts and 25,000,000 contacts.

Is the answer C? Looks like skinny tables goal is enhancing of report's and list view's performance