Mario is the default starting character in most Mario Kart games. As Nintendo’s mascot and the central character of the wider Mario franchise, he is typically the first racer highlighted or selected when a new Mario Kart game begins.
Mario has appeared in every main Mario Kart release, beginning with Super Mario Kart for the Super Nintendo Entertainment System in 1992. That original game included eight playable racers, but Mario served as the series’ natural representative and remained the most recognizable starting choice in later entries.
The other options are recurring racers, not the usual default. Luigi, Peach, and Yoshi have all been playable since the earliest game, while later titles expanded the roster with characters such as Bowser, Wario, Rosalina, and characters from other Nintendo series. “Default” can refer to menu selection or the first character shown, but Mario is the canonical answer in either sense for a general quiz question.