Which of the following TCP/IP protocol allows an application program on one…

Concept: In the TCP/IP suite, the transport layer offers two delivery styles. A connection-oriented byte-stream service (TCP) sets up a reliable, ordered connection before any data flows; a connectionless datagram service hands each self-contained message to the network independently, with no prior handshake. The protocol whose entire purpose is to let one application program ship a single datagram to another application program — addressed by port number, with minimal overhead and no connection setup — is the User Datagram Protocol (UDP).

Application: The defining specification (RFC 768) describes UDP exactly as the question phrases it: a procedure for application programs to send messages (datagrams) to other application programs with a minimum of protocol mechanism. UDP runs over IP, multiplexes to the right program using source and destination ports, and forwards each datagram on a best-effort basis without establishing or maintaining a connection. So the protocol that lets an application on one machine send a datagram to an application on another machine is UDP.

Contrast with the other choices:

• VMTP — a transport protocol designed for request-response (transaction/RPC) message exchange in distributed systems, not the standard TCP/IP datagram delivery service for ordinary application programs.

• X.25 — an older packet-switched WAN networking standard (network/data-link layers), not a TCP/IP transport protocol for application-to-application datagrams.

• SMTP — an application-layer protocol for transferring email between mail servers; it relies on an underlying transport, it does not itself provide the datagram delivery service.

Cross-check: Of the four, only UDP sits at the transport layer with a connectionless, datagram (message) delivery model addressed by ports — matching the description precisely. The correct answer is UDP.