Final answer:
The file used to customize Linux on first boot is 'cloud-init.txt', it comes from the 'cloud-init' package allowing automated system initialization in cloud environments.
Step-by-step explanation:
The file used to customize Linux on first boot is cloud-init.txt. This file is part of the cloud-init package, typically used for automating the initialization process of virtual machines after they are created in a cloud environment. With this tool, you can perform tasks like setting up users, installing packages, and writing files before the machine gets into the hands of the user.The correct answer is C - unattended.conf.
When customizing Linux on first boot, the file that is used is the unattended.conf file. This file contains instructions and settings that can be used to automate the installation and configuration process of the operating system. It allows administrators to define various aspects of the system, such as hostname, network settings, package selection, and more.By modifying the unattended.conf file before the first boot, administrators can ensure that the Linux system is customized according to their specific requirements.