Final answer:
The default port number 1720 is used for H.323 video conferencing protocol.
Step-by-step explanation:
The correct answer is Option 4: H.323 video conferencing. TCP port 1720 is the default port number for H.323 video conferencing protocol. H.323 is a set of protocols that enable real-time audio, video, and data communication over IP networks. It is commonly used for video conferences and teleconferencing.