Which OSI layer's protocols collaborate with UDP to emulate some of the reliability provided by TCP?

• A. Transport layer
• B. Network layer
• C. Application layer
• D. Data Link layer

Answer: (C)

Reliability can be added on top of UDP by higher-level protocols that define their own acknowledgments, retransmissions, sequencing, and integrity checks. Those mechanisms live at the application layer, where a protocol can tailor how to recover from loss for its specific use case. Since UDP itself is a connectionless, best‑effort transport, TCP’s reliability isn’t built in; instead, some application-layer protocols implement their own reliability over UDP to achieve similar outcomes. That’s why the correct layer is the Application layer. The network and data link layers don’t provide end-to-end reliability for UDP, and TCP’s reliability lives with the transport layer rather than collaborating with UDP.