What takes place when a planeswalker’s loyalty reaches 0?

When a planeswalker’s loyalty reaches 0, it is put into the owner's graveyard. This is a specific rule in Magic: The Gathering that applies to planeswalkers. When a planeswalker's loyalty counters drop to 0 due to being affected by damage or through using its loyalty abilities, it ceases to exist on the battlefield as a permanent and is then sent to its owner's graveyard as a state-based action.

This rule highlights the importance of managing loyalty when it comes to playing with or against planeswalkers. Unlike other permanents, which might have different fates upon reaching certain life thresholds (such as creatures dying), planeswalkers specifically have a mechanism that ensures they are removed from the game board once their loyalty is depleted. Therefore, the correct answer directly reflects this mechanic within the game.