What must be true about a card in order for it to transform?

For a card to transform, it must be a double-faced card. This means the card has two different faces with distinct abilities, one typically representing a creature in one state and another representing a different state or entity. The transformation mechanic in Magic: The Gathering is specifically designed for these double-faced cards, which have a condition under which they can be turned from one side to the other, such as meeting certain criteria or being affected by certain spells or abilities.

Understanding this, choices such as a requirement for the card to be a legendary creature, to be in a player's hand, or to have reached the battlefield do not directly pertain to the transformation mechanic. While some double-faced cards may be legendary or might need to be on the battlefield to meet specific conditions for transformation, these attributes are not universal requirements for all cards that transform. The key point is that the card must be double-faced in order to have the capability of transforming at all.