What should you do?

You have several SharePoint sites.

You plan to load a custom script in all pages of the sites. You need to ensure that you can activate or deactivate the script at the site level.

What should you do?

A.
Create a site definition and modify the CustomJSUrl attribute in the Onet.xml file.

B.
Create a site definition and modify the <system.web> element in the web.config file.

C.
Create a user control that contains the script. Create a Feature that overrides the ControlArea delegate control.

D.
Create a user control that contains the script. Create a Feature that overrides the AdditionalPageHead delegate control.

Explanation:
MNEMONIC RULE: AdditionalPageHead

The delegate control resides in the AdditionalPageHead control on the page. It registers some ECMAScript (JavaScript, JScript) on the page.

How to: Customize a Delegate Control
http://msdn.microsoft.com/en-us/library/ms470880.aspx