Which of the following keys can be pressed to exit less?

The 'q' key is the correct choice for exiting the 'less' command in a Linux environment. When you use 'less' to view files or command output, pressing 'q' allows you to immediately quit the viewer and return to the command prompt. This functionality is designed to be intuitive, making it easy for users to exit quickly without additional steps.

The other provided options either serve different purposes or are not intended for exiting 'less'. The ESC key is typically used to exit other interactive modes or commands but not specifically to quit 'less'. Ctrl+C is usually utilized to terminate a running command or process, which may not cleanly exit 'less' and instead can lead to an abrupt stop. The Space key is used to scroll down a page within the viewer but does not contribute to exiting the 'less' command. Therefore, 'q' is specifically designated as the exit command for 'less', making it the most appropriate choice for this scenario.