The BMC (Baseboard Management Controller) cable and IO Module in Supermicro systems like the SYS-421GE-TNHR2-LCC are often tied to add-on cards (AOC) within the PCIe tray. The AOC cage (likely an Add-On Card cage) must be removed first to access these components, as it houses cards and cables interfacing with the BMC. Supermicro’s design (e.g., SYS-420 series) places AOC cages as sub-assemblies within the PCIe tray, requiring removal before deeper access. A (AOM cage) may be a typo or misidentification, B (BMC cage) isn’t standard terminology here, and C (PCIe tray) is too broad since it’s the parent assembly. D fits Supermicro’s DLC service hierarchy, making it correct.

References: Supermicro PCIe tray sub-assembly guides, DLC system component access.