C is the right answer for the obvious reasons as specified by Artur above. Thanks!

Ignore the previous comment. It was for the previous question :-/

"C" is the correct answer.

"A" is the correct answer because of Lookup Data Skew.
If one account has more than 10,000 Cases looking up to the Account, it will cause locking when creating a new case. Splitting the account into smaller logical accounts and distributing the cases between a bank of these accounts would ensure that no more than 10,000 lookups occur against the same account.

The right answer is C - Have the integration create both contacts and their corresponding accounts; use an Account sharing rule to grant sales reps access to the contacts.
A is incorrect because of the data skew
B is incorrect because Contact Sharing rule do not applied for objects, controlled by Parent
D is incorrect because if contact isn’t linked to an account it's always private, regardless of your organization’s sharing model. Only the owner of the contact and administrators can view it. Sharing rules and workflow rules don’t apply to private contacts.

https://help.salesforce.com/s/articleView?id=000324018&type=1