The correct answer is D, :w filea.txt. This command will save the current file as filea.txt before exiting vi.
The syntax of the command is:
w [filename]
The colon (:) begins command-input mode, which allows the user to enter commands at the bottom of the screen. The w stands for write, which means to save the file. The filename is the name of the file to be saved.
If no filename is given, the command will save the file with the same name as the original file.
Therefore, the command :w filea.txt will save the current file as filea.txt and return to command mode. To exit vi, the user can then enter :q, which stands for quit.
The other commands are incorrect for the following reasons:
* A, %s filea.txt: This command will not save the file, but it will try to substitute the string filea.txt in the file. The syntax of the command is:
%s/pattern/replacement/
The % symbol means to apply the command to the whole file. The s stands for substitute, which means to replace a pattern with another pattern. The pattern is the string to be matched, and the replacement is the string to replace the matched string. However, this command is incomplete, because it does not have a replacement or a delimiter. The command will cause an error and will not save the file.
* B, %w filea.txt: This command will not save the file, but it will try to write the file to a filter. The syntax of the command is:
%w !command
The % symbol means to apply the command to the whole file. The w stands for write, which means to save the file. The ! symbol means to execute an external command. The command is the name of the filter to which the file is written. However, this command is incomplete, because it does not have a ! symbol or a filter name.
The command will cause an error and will not save the file.
* C, :save filea.txt: This command will not save the file, but it will try to save the file under a new name and switch to the new file. The syntax of the command is:
saveas filename
The saveas command is similar to the write command, but it also changes the current file name to the new file name. However, this command is incorrect, because it does not have the as keyword after save. The command will cause an error and will not save the file.
* E, :s filea.txt: This command will not save the file, but it will try to substitute the string filea.txt in the current line. The syntax of the command is:
s/pattern/replacement/
The s stands for substitute, which means to replace a pattern with another pattern. The pattern is the string to be matched, and the replacement is the string to replace the matched string. However, this command is incomplete, because it does not have a replacement or a delimiter. The command will cause an error and will not save the file.
:
How to Save a File in Vi / Vim Editor & Quit - phoenixNAP
How to Save a File in Vim / Vi and Quit the Editor | Linuxize
How to Save a File in Linux After Using the Vi Command - Computer Hope
How to Save a File in Vi / Vim Editor in Linux - Tecmint
How to Save a File in Vi / Vim Editor & Quit - GeeksforGeeks

The <, > and %> are valid stream redirection operators within Bash. The < operator redirects the standard input of a command from a file or another command. For example, sort < names.txt will sort the lines of the file names.txt and print them to the standard output. The > operator redirects the standard output of a command to a file or another command, overwriting the file if it exists. For example, echo "Hello World" > greeting.txt will write the string "Hello World" to the file greeting.txt. The %> operator redirects the standard error of a command to a file or another command, overwriting the file if it exists. For example, ls foo %> errors.log will list the contents of the directory foo and write any error messages to the file errors.log. The other options are either invalid or do not perform the desired task. The <<< operator is not a valid Bash operator, but it is used in some other shells like Zsh to redirect a string literal as the standard input of a command. The >>> operator is not a valid Bash operator, but it is used in some programming languages like Python to denote a bitwise right shift operation. References:
* LPIC-1 Exam 101 Objectives, Topic 103: GNU and Unix Commands, 103.4 Use streams, pipes and redirects
* LPIC-1 Linux Administrator 101-500 Exam FAQ, LPIC-1 Exam 101 Objectives, GNU and Unix Commands (Total Weight: 25)

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