According to me answer will be A and B.
Controller extension leverages the built-in-functionality of Standard Controller, but overrides one or more actions.
Where as, Custom Controller replaces the functionality of Standard Controller without leveraging it.
There is a significant difference in between leveraging and replacing.
Also, think about that, a class to be recognized as controller extension it must have a constructor that takes a single argument of type ApexPages.StandardController or custom controller you want to extend.
For more information, refer What are Custom Controllers and Controller Extensions?
https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_controller_def.htm