How do you accomplish this?

You want to allow multiple users the write access to files within the same directory, in
addition, you want all the new files created in this directory to be of the required group
instead of the primary ID of the user who creates the file. How do you accomplish this?

A.
Change the primary group ID of every user to the required group.

B.
Set the setgid bit on the directory.

C.
Change the group owner of the new files manually.

D.
Run a cron job to change the group owner.