Which layer divides each message into packets at the source and re-assembles…

Concept: Each OSI layer defines and manages its own protocol data unit (PDU). The Transport layer takes the application's message and performs segmentation, splitting it into transport-layer units — TCP segments (numbered and reassembled in order at the destination) or UDP datagrams (independent units delivered without ordering or reassembly guarantees) — while the layers below it only forward that unit hop-by-hop and never reconstruct the original application message.

Application: This question describes exactly that source-to-destination split-and-rebuild behaviour, so the layer being asked about is the Transport layer. The stem's word "packet" is used in the loose, everyday sense of "a piece the message was split into" — the precise transport-layer terms are segment (for TCP) and datagram (for UDP).

• Network layer: routes and forwards data hop-by-hop between networks using logical (IP) addressing, and may fragment a datagram to fit a link's MTU — but it never assembles pieces back into the original application message end-to-end.

• Data link layer: frames bits for transmission across a single physical link and checks for link-level errors; its scope is one hop, not source-to-destination.

• Physical layer: transmits raw bits as electrical, optical, or radio signals, with no concept of a message being divided or reassembled at all.

Answer: Transport layer.