Which one of the following is a connectionless protocol used in real-time streaming applications where best effort delivery is acceptable?

The correct choice is UDP, which stands for User Datagram Protocol. This protocol is connectionless, meaning it does not establish a dedicated end-to-end connection before sending data. Instead, it sends packets of data, known as datagrams, directly to the destination without the overhead of establishing a connection, making it efficient for real-time streaming applications.

In scenarios where timeliness is more important than guaranteed delivery, such as streaming audio or video, UDP is preferred. It allows for packets to be sent quickly, accepting that some may be lost or arrive out of order. Applications like online gaming, live broadcasts, and VoIP (Voice over Internet Protocol) benefit from UDP's speedy delivery, where minor data loss does not significantly affect the overall user experience.

The other options are designed for different purposes. Transmission Control Protocol (TCP) is a connection-oriented protocol that ensures reliability by establishing a connection and guaranteeing the delivery of packets. HTTP, while used for transferring web pages and data, relies on TCP for its transport method, therefore also being connection-oriented. File Transfer Protocol (FTP) is specifically designed for transferring files and is also connection-oriented, utilizing TCP for reliable delivery.