specification defining the PHY and MAC layer of low power devices.

The correct answer is: D. IEEE 802.15.4.lowpan

IEEE 802.15.4 is a low-power, low-rate wireless personal area network (WPAN) standard. It is a standard for the physical layer (PHY) and medium access control (MAC) layer of a wireless network. It is designed for use in low-power devices, such as sensors and actuators.

IEEE 802.15.4.lowpan is a sub-standard of IEEE 802.15.4 that defines the PHY and MAC layer for low-power devices. It is designed for use in networks of low-power devices, such as sensors and actuators.

The other options are not correct because they do not define the PHY and MAC layer of low power devices.

• A. IETF 6lowpan is a protocol that is used to connect IPv6 devices to IPv6 networks. It is not a PHY or MAC layer protocol.
• B. IETF CoAP is a lightweight application protocol for constrained devices. It is not a PHY or MAC layer protocol.
• C. RFID/NFC are technologies that are used to identify objects. They are not PHY or MAC layer protocols.