|A) It uses the public switched telephone network.||B) Skype is an example of VoIP software.|
|C) It uses a slow speed dial-up connection.||D) All of the above|
VOIP is an acronym for Voice Over Internet Protocol, or in more common terms phone service over the Internet.
Voice over Internet Protocol is a category of hardware and software that enables people to use the Internet as the transmission medium for telephone calls by sending voice data in packets using IP rather than by traditional circuit transmissions of the PSTN.
Here in the given options, Skype is the only voice service that enables us to talk over internet.