I am new to Linux and the question has also asked to provide the long option as the answer. This is a question from the HTB Academy free box, while solving I have tried many options like
but all the answers are getting rejected. Where am I making the mistake?
I know that su takes by default the root if not defined but when I need to define I need to provide the user's name. Following this I Have tried but with no success

you need to read the man page of the tool:
su [options] [username]
but I think you are looking for sudo (not su). And again, see man page about usage.

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su username -c "command"

will work to execute a command as the named user and requires knowing the password for that user.

sudo su username -c "command"

will do the same but only requires your password

It may be that the question is "how to switch to a different user (specify the long version of the option)" in which case

su --login username

will give you a clean login environment for that user and still requires knowing that user's
password.

Read the man pages for su and sudo to learn what options are available and what they do.

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just a comment: su and sudo should not be used together.
You need to know both and [obviously] need to select the proper one [in every case].

Should not possibly, but in some cases it may be necessary.

If I am doing a major reconfig of a system where I need to use sudo on many different consecutive commands and the root password has never been set, then rather that adding a root password and using su I would prefer to use sudo su for security reasons. I suspect others do the same at times.

Also if a user comes up to me with a problem I can use the "sudo su username -" to get directly into the users environment and mimic his situation to identify a fix without ever needing to know his password. Again, a security consideration.

While I would not encourage that practice in most cases it can be advantageous.

All use of sudo is routinely logged, as is use of su, so I don't really see a big problem there.

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I don't think so. I would rather say: a properly configured sudoers file is sufficient.
I can't see any security reason or any advantage to do that (except the incorrect sudoers file or laziness).
no, it is not a big problem, you are right. Bad practice (and probably a security hole too).

Except when needing to do a lot of commands as root in a single session on a system that has no root password assigned, or for which you have sudo permissions but not the root password, you are probably correct.
In that situation entering "sudo su" once then the rest of the commands while omitting the sudo is much more keystroke efficient (and easier on the carpal tunnel tendons).

"ssh username@0" does the same thing. Back in more trusting times -- and before it became the Wrong Thing To Do -- so did "telnet 0 username".

Because of less overhead and cleaner logging you should do

sudo -su username
sudo -su username command

rather than

sudo su username
sudo su username -c "command"

and

sudo -iu username
sudo -iu username command

rather than

sudo su - username
sudo su - username -c "command"
(where the traditional "-" works like the GNU "--login")

Which "option" needs to be set to "execute a command" as a different user using the "su" command? (long version of the option)

--command

Ok, just first get your credentials on the desktop to know your password (remember the password is invisible on linux console)

then execute:

$su
(tipe your password)

then

#su --help -> here are the su's options and your answer (choose the long one)

u're welcome to the future students.

I know it's not the direct answer but you need to learn how to solve this kind of problems to achieve your goals.