Which protocol operates at Layer 4 and is used for reliable, connection-oriented communication?

Correct answer: TCP

Option C is correct because TCP (Transmission Control Protocol) operates at OSI Layer 4 (Transport layer) and provides reliable, connection-oriented communication through a three-way handshake, sequencing, acknowledgments, and retransmission of lost segments. Option A, IGMP (Internet Group Management Protocol), operates at Layer 3 and is used for managing multicast group memberships, not for reliable unicast data transport. Option B, ICMP (Internet Control Message Protocol), also operates at Layer 3 and is used for diagnostic and error-reporting purposes such as ping and traceroute, not for application data transfer. Option D, UDP (User Datagram Protocol), does operate at Layer 4 but is connectionless and unreliable, providing no guaranteed delivery or sequencing.