Which User Account Control (UAC) setting must be defined in the Default Domain Policy?

The correct answer is focused on enabling the secure desktop for elevation prompts as part of the User Account Control (UAC) settings defined in the Default Domain Policy.

When a user requires elevated privileges to perform specific tasks, User Account Control uses a secure desktop to prevent other applications from interfering during the elevation process. This secure desktop ensures that users can clearly see the elevation prompt and that malicious software cannot manipulate or present fake prompts to trick users into granting higher privileges.

Implementing this setting in the Default Domain Policy is essential for maintaining a secure environment across all user accounts within the domain. It provides a uniform method for controlling how elevation prompts are handled, ensuring that users remain aware of elevation requests and reducing the risk of unauthorized access.

All other options do not focus on the core security requirement of maintaining a clear and secure elevation process, and some may even compromise security instead of enhancing it.