B. unlimited failure reports for either a work order or any of its tasks
This is incorrect. The standard functionality enforces a one-to-one relationship; you cannot create multiple, separate failure reports for the same work order header or the same task.
C. unlimited failure reports for each work order, but none for a task
This is incorrect because failure reports can be created at the task level, and the system does not permit an unlimited number of reports for the work order header.
D. just one failure report
This is too restrictive. While only one report can be made for the work order header, additional reports can be created for each of its associated tasks.