Which FC mode is the device MDS-9148S-2 operating in?

FDISC (Frame DISC) is a specific login service message and part of the N-Port login process, not an operating mode for a Fibre Channel switch.

The Cisco MDS 9148S-2 is a Fibre Channel switch that is capable of operating in N-Port Virtualization (NPV) mode. In NPV mode, the switch acts as an aggregation device for end devices, presenting them to an upstream core Fibre Channel fabric as virtual N-ports, thereby simplifying the fabric design and conserving domain IDs.

NPIV (N-Port ID Virtualization) is a feature that allows a single physical N-port to register multiple N-port IDs, and while NPV uses NPIV, it is not the operating mode of the switch itself.

FPMA (Fibre Channel Port Mapping Agent) is not a standard or recognized operating mode for Fibre Channel switches.