Which file appears on-screen and is available at the user's request?

The correct choice is "Read Me." A "Read Me" file is typically designed to provide information to users about software, including installation instructions, features, and important notes. This file is often displayed on-screen and can be accessed whenever the user chooses, making it user-friendly for quick reference.

"Online help" and "help files" generally refer to documentation or resources that provide assistance and guidance while using a software application but may not always be immediately visible on the screen. Users typically access them through menus or help buttons rather than having them displayed directly upon installation or launching the application.

"Service packs," on the other hand, are updates or patches released to fix bugs or improve functionality but do not serve as on-screen files available at a user's request. They are mainly intended to be downloaded and installed rather than constantly accessible for user reference.

Thus, the "Read Me" file stands out because of its intention for immediate visibility and easy access by the user.