Which component is upgraded first during a PowerFlex cluster upgrade?

The order of operations for a PowerFlex upgrade is strict and dictated by the software architecture.

Metadata Managers (MDM): The MDM is the "brain" of the cluster. It holds the cluster map and configuration. It must be upgraded first (Primary, then Secondary, then Tie-Breakers) so that it can understand the newer protocol versions and manage the upgrade of the subsequent components. An older MDM cannot manage a newer SDS.

Storage Data Servers (SDS): Once the MDMs are upgraded, the storage nodes are upgraded one by one (or fault set by fault set).

Storage Data Clients (SDC): Finally, the host-side drivers are updated.

Therefore, A (Metadata Managers) is the correct first step. Fault Sets (C) are logical groups, not software components to be upgraded.