Which of the following is a connectionless transport layer protocol?

TCP (Transmission Control Protocol) is a connection-oriented transport layer protocol that establishes a reliable, ordered, and error-checked connection between two hosts.

HTTP (Hypertext Transfer Protocol) is an application layer protocol used for distributed, collaborative, hypermedia information systems, typically running over TCP.

SMTP (Simple Mail Transfer Protocol) is an application layer protocol used for sending email, typically running over TCP.

UDP (User Datagram Protocol) is a connectionless transport layer protocol that provides a best-effort, unreliable delivery service without establishing a connection prior to sending data. It operates by sending individual datagrams without guarantees of delivery, ordering, or duplicate protection, making it suitable for applications where speed is more critical than reliability.