A FlashSystem 840 attaches to a Power E870 via:

A.
16Gb Fibre Channel adapter.

B.
8Gb GX++ adapter.

C.
large write cache SAS adapter.

D.
Optical CXP converter adapter.

Explanation:
The IBM FlashSystem 840 supports three different protocol interface cards:
Fibre Channel (16 Gbps, 8 Gbps, and 4 Gbps)
Fibre Channel over Ethernet (FCoE) (10 Gbps)
InfiniBand QDR (40 Gbps)
iSCSI (10 Gbps)
Implementing IBM FlashSystem 840 (July 2015), page 26
http://www.redbooks.ibm.com/redbooks/pdfs/sg248189.pdf