What effect does a spell or ability that instructs a transformation have on a double-faced card?

A spell or ability that instructs a transformation specifically targets double-faced cards, which have two different printed sides but exist as a single card in the game. When such a spell or ability is applied, it allows the card to reveal or show its other side, effectively transforming it from one side to the other.

This transformation is an inherent function of double-faced cards, which are designed to transition between their two forms. For example, a card like "Garruk Relentless" can transform into "Garruk, the Veil-Cursed," and this change is executed through the correct game mechanics as indicated by the transformation instruction.

In contrast, the other options do not reflect the rules governing transformations for double-faced cards. Returning the card to the player's hand, destroying it, or reducing its power are actions that do not relate to the fundamental transformation mechanism that allows the card to change sides. Thus, the understanding of how transformations work for double-faced cards is essential, and recognizing that they can switch to show their other side as a direct result of a spell or ability is key to mastering MTG Comprehensive Rules regarding this type of card.