Multiple I/O modules must be installed in Dell Unity XT 680 storage.

The Unity XT 680 system supports four types of I/O modules: 12 Gb/s SAS back-end, 4-Port 16 Gb Fibre Channel, 4-Port 25 Gbps Ethernet, and 4-Port 10 Gbps Ethernet. The 12 Gb/s SAS back-end module provides connectivity to the disk drives and expansion enclosures, while the other three types of modules provide network connectivity to the hosts and clients. The Unity XT 680 system has six I/O module slots, three for each SP. The 12 Gb/s SAS back-end module must be installed in slot 0 of each SP, and the other modules can be installed in slots 1 and 2 of each SP. Therefore, the correct answer is D. 12 Gb/s SAS back-end. References:

Dell EMC Unity XT: Hardware Information Guide

Dell EMC Unity XT: Installation Guide