Historical Rules · 18 June 2010
Rule 204.3d
Official source text from the 18 June 2010 Rules release
If an effect instructs a player to choose a subtype, that player must choose one, and only one, existing subtype, and the subtype he or she chooses must be for the appropriate card type. For example, the player can't choose a land type if an instruction requires choosing a creature type. Example: When choosing a creature type, "Merfolk" or "Wizard" is acceptable, but "Merfolk Wizard" is not. Words like "artifact," "opponent," "Swamp," or "truck" can't be chosen because they aren't creature types.