Historical Rules · 1 October 2009
Rule 802.2a
Official source text from the 1 October 2009 Rules release
Any rule, object, or effect that refers to a "defending player" refers to one specific defending player, not to all of the defending players. If the source of an ability that refers to a defending player is an attacking creature, it will usually refer to the player it's attacking or the controller of the planeswalker it's attacking. If there are multiple defending players that could be chosen, the controller of the ability chooses one. Example: Rob attacks Alex with Runeclaw Bear and attacks Carissa with a creature with mountainwalk. Whether the creature with mountainwalk is unblockable depends only on whether Carissa controls a Mountain.