The Nintendo accessory required for Versus Mode in the Game Boy Color version of Tony Hawk's Pro Skater was the Game Link Cable.
The Game Boy Color adaptation included a human-versus-human mode that ran on a single course. Two players needed compatible Game Boy systems, two copies of the game and a cable connecting the handhelds. The cable let the cartridges exchange multiplayer data during the race.
The game's Rival Mode can cause confusion because it resembles Versus Mode but uses a computer-controlled opponent. Versus Mode is the human multiplayer option, which is why the Game Link Cable is specifically required. The manual refers to this feature as Link Mode in its multiplayer instructions.
The other choices belong to different Nintendo accessories. The Transfer Pak connected certain Game Boy cartridges to Nintendo 64 games, the Rumble Pak added vibration, and the Expansion Pak increased Nintendo 64 memory. None of those accessories provided the Game Boy Color connection needed here.