Explanation
To achieve the setup of sharing the same SFTP directory across each of their Marketing Cloud accounts, NTO should copy the parent SFTP user into each child business unit. This will allow each business unit to access the same SFTP directory as the parent business unit and share files among them. The other options are incorrect because:
Child business unit SFTP user should be created. This will create a separate SFTP user and directory for each child business unit, which will not allow them to share files with the parent business unit or other child business units.
Each business unit should have multiple SFTP users. This will create unnecessary complexity and confusion for NTO, as they will have to manage multiple SFTP users and directories for each business unit. This will also not allow them to share files among different business units easily.
All child business units should have an individual SFTP user. This is similar to the first option, as it will create a separate SFTP user and directory for each child business unit, which will not allow them to share files with the parent business unit or other child business units.