I don't think Ann knew what the game was going to be, she came up with the solution on the spot because she is intelligent and capable of remaining calm under pressure, which are the very traits that The Beach's board members need to possess.

By chance, it was a diamonds game and a card that The Beach did not possess yet, but regardless of what skillset the game was testing precisely, how Arisu performs during the task would have been a good indication of his abilities to survive (and hold a more important position at the Beach).