What command line will create the user falco with home directory assigned to the group users as primary group?

Option A uses -g to set the primary group correctly but omits -m, so no home directory is created for the user.

The -f flag in useradd sets the number of days after a password expires before the account is disabled, not the primary group.

The -m flag instructs useradd to create the user's home directory if it does not exist, and the -g flag sets the primary group by name or GID. Together, 'useradd -m -g users falco' satisfies both requirements stated in the question.

'add user' is not a valid Linux command; the correct utility is useradd (or adduser on some distributions with different syntax).

'add -user' is not a valid command or option syntax on any standard Linux distribution.