When can a player planeswalk?

A player can planeswalk primarily when the game state allows for it through specific abilities or events. The correct choice addresses these mechanics accurately by identifying that a player may planeswalk as a result of a planeswalking ability or if the owner of the face-up plane card leaves the game.

In context, this means that if a card has an effect that allows a player to switch to a different plane, or if the conditions are met for a planeswalker to affect the game state, planeswalking can occur. An example of a planeswalking ability would be any ability written on a card that has planeswalking effects, such as certain enchantments or abilities of special planeswalkers.

Other options do not accurately represent these mechanics. For instance, suggesting that planeswalking can only happen during a specific phase or as a result of certain game actions does not encompass the comprehensive rules surrounding planeswalking and limits the player’s understanding of when and how this action can occur in the game. Thus, option C encapsulates the rules regarding the activation of planeswalking correctly.