What must happen when shuffling a library?

When shuffling a library, the primary objective is to randomize the order of the cards. The correct answer reflects the rule that cards must be randomized thoroughly to ensure a fair game. This randomization helps to maintain the integrity of the game by preventing players from knowing the order of their cards, which could lead to an unfair advantage.

In tournament settings, this requirement for thorough randomization is crucial to uphold the principles of fair play and competition. The intent behind this rule is to ensure that no player can predict the top card of the library or any sequence of cards, which is essential in a game that relies heavily on chance.

Maintaining the order of all cards, requiring player observation, or only shuffling selected cards would undermine the fairness of the game and does not comply with the rules established for shuffling a library. Thoroughly randomizing the cards is the key action necessary to create a level playing field during gameplay.