Answer should be B :

This approach allows you to extend the standard controller's capabilities, adding custom logic for the merge functionality. By using a controller extension, you can access the standard controller methods (like saving a record) and add your custom methods (like merging records). This is the most appropriate choice for adding specific functionalities while still using the built-in capabilities of the standard controller without building a new from scratch controller for save and merge !

I think correct answer is B also. Because we need to merge the record s we wil need an extention controller.

Answer is B.