To solve this question, the user needs to have some understanding of ASP.Net development and its user interface.

The correct answer for this question is option B: System.UI.Naming.Container.

Explanation of each option:

A. AppDomainsetup: This is an option for setting up the application domain for an application. It is not related to creating unique Id's in ASP.Net.

B. System.UI.Naming.Container: This is the correct option as it is the interface used by ASP.Net to create Unique Id's. In ASP.Net, controls rendered on the server are assigned a unique ID attribute that is used to identify the control in the HTML markup.

C. IAsyncResult: This is an interface that is used to manage the status of an asynchronous operation. It is not related to creating unique Id's in ASP.Net.

D. customFormatter: This is a formatter that can be used to format data to be displayed on a web page. It is not related to creating unique Id's in ASP.Net.

Therefore, the answer is: B. System.UI.Naming.Container.