What is the High Density Storage (HDS) Option kit?

A.
a kit convert the standard DDR3 DIMM slots to the new DDR4 DIMM slots

B.
a kit contains two 2.5-inch SATA or SAS hard to install in the interior of an HP Z840 workstation

C.
two HDS cage modules that convert two 3.5-inch drive bays into four 2.5-inch bays

D.
a kit to convert the standard HP Z840 workstation to rack mountable system