You use a third-party site definition to create SharePoint sites.

You need to add a Web Part to the home page of the site definition.

Which file should you modify?

A.
default.master

B.
Onet.xml

C.
Sp.xml

D.
web.config

Explanation:
MNEMONIC RULE: “third-party site definition = Onet.xml”

You can perform the following kinds of tasks in a custom Onet.xml file that is used for either a custom site definition or a custom web template:

* Specify an alternative cascading style sheet (CSS) file, JavaScript file, or ASPX header file for a site definition.
* Modify navigation areas for the home page and list pages.
* Add a new list definition as an option in the UI.
* Define one configuration for the site definition or web template, specifying the lists, modules, files, and Web Parts that are included when the configuration is instantiated.
* Specify Features to be included automatically with websites that are created from the site definition or web template.

Understanding Onet.xml Files
http://msdn.microsoft.com/en-us/library/ms474369.aspx