I think it's B and here is why :

A custom controller extension allows you to extend the functionality of the standard controller. You can use the standard save method and add custom logic to merge records as needed. This approach leverages both the built-in features of the standard controller and the flexibility of custom Apex code.
While a custom controller could also meet the requirement, it involves more work since you would need to implement all basic operations (like saving records) from scratch. Using a controller extension is more efficient as it builds on top of the standard controller.