A brief explanation why…
Starting with the original Donkey Kong in 1981 (Mario's first appearance), the Mario franchise has sold more than 528 million copies, with 'Super Mario' titles making up 311 million of that. It wasn't listed in the possible answers, but Tetris is close behind with roughly 500 million sales courtesy of its popularity as a download among mobile phone owners.
In third place with about half as many unit sales, some 280 million copies of Pokémon have been sold since the 1996 Japanese release of the original Pokémon RPG for Game Boy.
Grand Theft Auto and Call of Duty are roughly tied at 250 million despite the former being five years younger (1997 vs 2003), while Madden NFL has been around since 1988 and is surprisingly far down the list behind The Sims, Need for Speed, Final Fantasy as well as Minecraft and Wii Sports.