The correct answer is D. VPN.
An open standard is a technical standard that is publicly available and can be implemented by anyone without paying a license fee. Bluetooth, WWW, and HTML are all open standards. VPN is a private network that is created over a public network, such as the internet. VPNs are not open standards because they are typically proprietary technologies that are owned by a single company or organization.
Bluetooth is a short-range wireless technology that allows devices to communicate with each other. It is used in a variety of devices, including smartphones, laptops, and speakers. Bluetooth is an open standard, which means that it is freely available for anyone to use.
The World Wide Web (WWW) is a system of interlinked hypertext documents accessed via the Internet. It is also known as the Web or the Internet. The WWW was invented in 1989 by Tim Berners-Lee at CERN, the European Organization for Nuclear Research. The WWW is an open standard, which means that it is freely available for anyone to use.
HTML is a markup language that is used to create web pages. It is a standard that is maintained by the World Wide Web Consortium (W3C). HTML is an open standard, which means that it is freely available for anyone to use.
A virtual private network (VPN) is a private network that is created over a public network, such as the internet. VPNs are used to provide a secure and private connection between two or more devices. VPNs are not open standards because they are typically proprietary technologies that are owned by a single company or organization.