D.
Although letters may be informal as well, commercials tend to be the most informal.
https://www.bbc.com/news/technology-44640959
https://uxdesign.cc/the-psychology-of-why-social-media-is-so-addictive-67830266657d
https://www.linkedin.com/pulse/social-media-manipulation-human-behaviour-profit-john-paul-drake
i think the answer would be - The longing for connection
