What is the consequence of each step or phase ending?

The consequence of each step or phase ending is that any remaining mana in a player's mana pool empties. This is an important rule in Magic: The Gathering because it ensures that players need to use or spend their mana before a step or phase concludes. After a phase or step ends, the mana that has not been used is lost, which emphasizes the importance of planning and resource management during gameplay.

The other choices represent actions that do not occur at the end of every step or phase. For example, while players do indeed shuffle their libraries at certain moments (like when a card instructs them to do so), this does not happen automatically at the end of each step or phase. Similarly, untapping all tapped creatures occurs only during the untap step of a turn, and players do not discard down to their maximum hand size at the end of a step or phase unless a specific rule or effect dictates such an action after a certain event, such as having to discard at the end of a turn.