Which card file is required for all personnel assigned to the unit?

The correct answer is DA Form 3955, which is a vital document used to maintain an updated record of personnel assigned to a unit. This card file serves the purpose of tracking changes in duty assignments, ensuring that all personnel movements are documented and readily accessible. It facilitates effective management of personnel records, particularly in military contexts, where it is crucial to have accurate and immediate access to information regarding unit members.

In contrast, the other forms listed serve different purposes: DA Form 3960 is used for personnel accountability during emergencies or changes in unit status, DA Form 3975 pertains to investigations and does not track personnel assignments, and DA Form 3985 is related to personnel records management but is not specifically tailored for tracking assignments within a unit. Therefore, DA Form 3955 is the essential form required to effectively monitor and manage the personnel assigned to a unit, highlighting its importance in maintaining organizational integrity.