Tandy 1000
Commodore 64/128
Apple II
IBM Personal Computer
 
Correct Answer: Commodore 64/128

A little background…

Before eventually becoming available for other systems, Quantum Computer Services began offering its Quantum Link software to owners of the Commodore 64 and 128 PCs in November 1985. Q-Link was based on a modified version of PlayNET (another online service) and provided features including program downloads, email, IM, chat rooms, forums, multiplayer gaming, music streaming, digital photos, as well as services for shopping, travel reservations, auctions and information sources including news, magazine and an encyclopedia.

Customers connected to Q-Link through dial-up modems with speeds ranging from 300 to 2400 baud, with 1200 baud being the most common. The service was normally open weekday evenings and all day on weekends with pricing set at $9.95 per month, plus six cents per minute (later raised to eight) for "plus" areas, which included most of the mentioned services. Users had an hour of free "plus" usage each month while folks hosting forums and trivia games could earn extra time.

By 1988, Quantum Computer Services worked with Apple and Tandy to provide versions of the software (AppleLink and PC Link) that would work on their machines, while IBM-compatible PCs gained access to Q-Link around the same time.

In 2005, Commodore enthusiasts reverse-engineered Q-Link and created the clone "Quantum Link Reloaded" which runs over the Internet instead of phone lines (if you don't have original hardware, you can access it using the VICE Commodore 64 emulator for PC or Linux).

In February 2017, Habitat developer Randy Farmer began an open-source effort at NeoHabitat.org to revive the early Q-Link graphical MUD.