Which of the following protocols is connection-less and does not ensure reliability of transmission?

The User Datagram Protocol (UDP) is indeed the correct choice because it is designed as a connectionless protocol that does not establish a dedicated end-to-end connection before sending data. This means that it does not guarantee the delivery of packets, nor does it provide any mechanisms for retransmission in case of lost packets or errors. As a result, UDP is often used in applications where speed is more critical than reliability, such as video streaming, online gaming, or voice-over-IP communications.

In contrast, the other protocols listed have characteristics that ensure reliability or establish a connection before transmitting data. Hypertext Transfer Protocol (HTTP) is built on top of Transmission Control Protocol (TCP), which is connection-oriented and ensures reliable delivery of data packets. Similarly, TCP guarantees that packets arrive in order and without errors by implementing error-checking mechanisms and retransmission capabilities as needed. Simple Mail Transfer Protocol (SMTP) is also based on TCP, thus inheriting the same reliability features. Therefore, these protocols are not considered connectionless and are designed to ensure that data transmissions are reliable and orderly.