The correct answer is: D. IEEE 802.15.4
IEEE 802.15.4 is a low-power, low-rate wireless personal area network (WPAN) standard. It is the lowest level of the ZigBee software stack, and provides the physical and medium access control (MAC) layers.
IEEE 802.11 is a family of wireless networking standards that are commonly used for local area networks (LANs). It is a higher-level standard than IEEE 802.15.4, and provides the physical, MAC, and data link layers.
IEEE 802.15.3 is a high-rate WPAN standard. It is a higher-level standard than IEEE 802.15.4, and provides the physical, MAC, and data link layers.
ZigBee is a wireless mesh networking protocol that is designed for low-power, low-bandwidth applications. It is based on the IEEE 802.15.4 standard, and provides the physical, MAC, network, and application layers.