(i) Red Hat
(ii) Ubuntu
(iii) Cent OS
(iv) Linux Mint

Open source refers to a program or software in which the source code is available to the general public for use and/or modification from its original design, free of charge.

By choosing an option logout through the session indicator on the far right side of the top panel.

The server distribution in Linux OS:
(i) Ubuntu Linux
(ii) Linux Mint
(iii) Arch Linux
(iv) Deepin
(v) Fedora
(vi) Debian
(vii) Centos

Advantages:
(i) It is free of cost
(ii) Accessible to everybody.
(iii) Not belongs to particular vendor.
(iv) It can be modified, for requirements.

(i) An operating system (OS) is a software program that enables the computer hardware to communicate and operate with the computer software.
(ii) It also acts as an interface between the user and the computer hardware and controls the execution of all kinds of programs.

The most popular operating systems are as follows:
(i) Windows 7 is the most popular operating system for desktop and laptop computers.
(ii) Android is the most popular smart phone operating system.
(iii) iOS is the most popular tablet operating system.

Difference between Thunderbird and Firefox in Ubuntu:
(i) Thunderbird is inbuilt email software of ubuntu whereas Firefox is a web browser.
(ii) Thunderbird is used to access email such as Gmail, hotmail whereas Firefox is used to open web pages including e-mail.

(i) Ubuntu has less hardware support for commercial/industrial/medical/ logistical therefore its less voted for use in big time backends.
(ii) Ubuntu doesn't support middlewares such as C panel, cloud linux and a plethora of other infrastructure or monitoring tools.
(iii) Hard to install graphic drivers especially for old hardwares. It is not possible to play the modern games, because of poor graphics quality.
(iv) The user switching from windows will not like the user experience on Ubuntu and will have difficulty in operating the OS.
(v) Ubuntu is not capable of playing MP3 files by default.

Copying Files and Folders to removable disk:
There are several methods of transferring files to or from a removable disk:
(i) Copy and Paste
(ii) Send To
METHOD I - Copy and Paste
(i) Plug the USB flash drive directly into an available USB port.
(ii) If the USB flash drive or external drive folder does NOT open automatically, follow these steps:
(iii) Click → Start Computer.

(iv) Double-click on the Removable Disk associated with the USB flash drive.

(v) Navigate to the -folders in your computer containing files you want to transfer. Right-click on the file you want to copy, then select Copy.

(iv) Return to the Removable Disk window, right-click within the window, then select Paste.

METHOD II - Send To
(i) Plug the USB flash drive directly into an available USB port.
(ii) Navigate to the folders in your computer containing files you want to transfer.
(iii) Right-click on the file you want to transfer to your removable disk.
(iv) Click Send To and select the Removable Disk associated with the USB flash drive.

The important functions of an operating System.
(i) Memory Management
(ii) Processor Management
(iii) Device Management
(iv) File Management
(v) Security
(vi) Control over system performance
(vii) Job accounting
(viii) Error detecting aids
(ix) Coordination between other software and users

In Ubuntu OS:
(i) Create a file: By right clicking in the desktop and also files be created by using file menu.
(ii) Delete a file: By using right-click choosing a move to trash or by using the menu.
(iii) Rename a file: By using right-click and choosing rename option.
(iv) Save a file: Press ctrl + x or F2 to exit. You will. then be asked if you want to save. (or) Press ctrl+ 0 or F3 and ctrl + x or F2 for save and exit.
In Windows OS:
(i) Create file: Open an application and create by using file menu.
(ii) Delete a file: By right click on a file and choosing the delete option to delete a file.
(iii) Rename a file: By right click on a file and choosing rename option to rename a file.
(iv) Save file: Press ctrl+S or file - save to save the file.