Duplicate Rules are available for all objects except which one?

Answer options:

A.Contacts
B.Lead
C.Accounts
D.Cases

Correct Answer: D 
Duplicate rules don’t exist for cases. A duplicate rule defines what happens when a user views a record with duplicates or starts creating a duplicate record. Salesforce provides standard duplicate rules for business and person accounts, contacts, and leads. You can also create duplicate rules. 
Other options are not correct because duplicate rules exist for them. 
Reference: 

https://success.salesforce.com/ideaView?id=08730000000DjN5AAK