Final answer:
OSPF is the routing protocol listed among the options, which includes OSPF, IPv4, IPv6, and TCP.
Step-by-step explanation:
The routing protocol among the options provided is a. OSPF (Open Shortest Path First). OSPF is a widely used internal gateway protocol (IGP) developed for IP networks based on shortest path routing. In contrast, b. IPv4 (Internet Protocol version 4) and c. IPv6 (Internet Protocol version 6) are protocols designed for use on packet-switched networks that route traffic based on the destination IP address but are not, themselves, routing protocols. Lastly, d. TCP (Transmission Control Protocol) is a core protocol of the Internet protocol suite that ensures reliable, ordered, and error-checked delivery of a stream of data between applications.