How does the game determine who becomes the defending player after the beginning of combat step?

The correct answer demonstrates that the defending player is determined by the active player’s choice during the combat phase of the game. In Magic: The Gathering, the active player initiates combat, which involves declaring attackers and subsequently determining who the defending player will be for that combat.

When an active player declares attackers, the defending player is simply the player who is not active. Thus, the identification of the defending player is not arbitrary; it follows the basic rules of turn structure where the player not actively taking the turn is always the defender. In this context, the active player does not make a random choice, nor do the players mutually agree on who the defending player is – it is a straightforward determination based on the active player’s state in the turn order.

This reflects the structured nature of Magic's play to ensure clarity and consistency in determining roles during combat, avoiding any potential confusion or disputes about who is involved in the combat interactions.