The correct answer is A. Instant Messaging.
Instant messaging is a text-based communication method between two or more people that is typically carried out over the Internet using instant messaging software. Instant messaging is often used for quick, informal conversations. It is a real-time communication method, which means that messages are sent and received immediately.
E-mail is a method of exchanging digital messages from one computer user to another over a computer network. E-mail is a text-based communication method, but it can also include attachments such as images, documents, and spreadsheets. E-mail is not a real-time communication method, which means that messages are not sent and received immediately.
Usenet is a global discussion system that consists of thousands of newsgroups. Newsgroups are organized by topic, and users can post messages to newsgroups and read messages that have been posted by other users. Usenet is not a real-time communication method, which means that messages are not sent and received immediately.
A mailing list is a list of email addresses that are used to send messages to a group of people. Mailing lists are often used to distribute information to a group of people who are interested in a particular topic. Mailing lists are not a real-time communication method, which means that messages are not sent and received immediately.