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

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
Quote:

-user -username

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