What is an 'SFP' (Small Form-factor Pluggable) transceiver and what flexibility does it provide compared to fixed-interface switches?

• A.An SFP is a hot-swappable modular transceiver inserted into an SFP cage on a switch or router, allowing the physical layer interface to be selected (copper 1000Base-T, multimode fiber 1000Base-SX, single-mode fiber 1000Base-LX, etc.) without replacing the switch itself; SFP+ supports 10 Gbps, QSFP supports 40 Gbps, QSFP28 supports 100 Gbps
• B.An SFP is a power supply unit installed in managed switches for PoE capability
• C.An SFP is a software plugin that adds routing features to a Layer 2 switch
• D.An SFP is a security certificate required for 802.1X port authentication