Which layer of the OSI model is associated with the reliable transmission of datagrams?

'Datagram' is a packet type or concept used at the Network layer; it is not a layer in the OSI model.

'Routing' describes a function performed at Layer 3; it is not an OSI layer name.

The Network layer (Layer 3) handles logical addressing and path determination but provides only best-effort, unreliable delivery with no guarantee of ordered or error-free receipt.

The Data Link layer (Layer 2) handles framing, MAC addressing, and error detection on a single link segment, not end-to-end reliable delivery across a network.

The Transport layer (Layer 4) provides reliable transmission through mechanisms such as sequencing, error detection, flow control, and acknowledgment-based retransmission; TCP specifically operates at this layer to guarantee ordered, error-free delivery of segments between hosts.

'Transmission' is not a layer in the OSI model.

The Session layer (Layer 5) manages the establishment, maintenance, and termination of communication sessions between applications, not reliability of data delivery.