Can a player change cards in their main deck between games?

The correct choice illustrates a fundamental rule regarding deck management during Magic: The Gathering tournaments. Between games, players are allowed to adjust their main decks by swapping cards from their sideboard. This is an important strategic element, as it allows players to tailor their decks to their opponent's strategy based on what they experienced in the previous game.

When a player finishes a game, they have the opportunity to look at their sideboard (the additional cards set aside for strategic adjustments) and make substitutions in accordance with the rules. This means they can bring specific cards in from their sideboard that might better suit their needs for the next game against their opponent. However, it's essential that players respect the deck size limits—maintaining a total of 60 cards in their main deck during constructed formats is necessary.

In contrast, swapping cards back and forth between the main deck and sideboard should not involve any unauthorized changes, such as reshuffling cards that are not part of this switching process. This enhances the integrity of the game and ensures that both players have a clear understanding of the expected configuration of decks for each match. Thus, the option that allows players to swap cards from their sideboard into their main deck captures this rule accurately.