Which statement is true regarding all double-faced cards?

Double-faced cards have the unique mechanic that allows them to represent two different sides, each with its own characteristics and abilities. Transforming a double-faced card means that you change it from one side to the other, typically based on specific conditions outlined on the card.

The correct choice notes that double-faced cards can only transform when instructed by a spell or ability. This is a fundamental aspect of how these cards operate within the rules of Magic: The Gathering. For instance, a werewolf card might transform when certain conditions are met, such as changing from day to night or vice versa in gameplay, aligned with the game's mechanics.

In contrast, the other choices do not accurately represent the rules surrounding double-faced cards. They cannot transform at any time (as option A suggests); rather, they require specific conditions. The statement that they do not have any effects (option C) is incorrect because these cards can have significant effects on gameplay based on which side is active. Lastly, stating that they remain the same unless discarded (option D) is misleading, as they can transform under certain circumstances before being discarded; the action of being discarded does not directly pertain to their ability to transform.

Thus, the assertion regarding their transformation relies on the ability provided by