Which step must the active player complete before declaring attackers?

Before declaring attackers, the active player must complete the beginning of combat step. This step is a crucial part of the combat phase in Magic: The Gathering. During the beginning of combat step, players are given an opportunity to cast spells and activate abilities before moving forward with declaring attackers. This is the phase where both players can respond to any potential threats or prepare their strategies before the active player declares which of their creatures will attack, influencing the upcoming combat.

The combat damage step and the declare blockers step occur after attackers have already been declared, making them irrelevant to this question. Similarly, the untap step is part of the beginning phase of a player's turn, not specifically tied to the actions taken during combat. Hence, completing the beginning of combat step is essential for the active player to properly engage in the combat phase and declare their attackers.