Final answer:
DHCP uses the UDP protocol at the Transport layer for efficient, connectionless data transmission to assign IP addresses to devices in a network.
Step-by-step explanation:
The protocol that DHCP uses at the Transport layer is UDP (User Datagram Protocol).
DHCP, which stands for Dynamic Host Configuration Protocol, operates using a connectionless method of data transmission in the network which is provided by UDP. This means that when a device connects to a network and needs an IP address, DHCP can assign one without the need to establish a persistent connection, which is something that TCP would require. As such, DHCP's use of UDP allows for quick and efficient communication that does not require the overhead of connection setup and maintenance that TCP entails.
Therefore, the protocol used by DHCP at the Transport layer is not IP, ARP, or TCP, but specifically UDP for its transaction-oriented processes.