Under what circumstances can something be "set in motion"?

A scheme card can be "set in motion" specifically in the context of an Archenemy game. In such games, the player designated as the Archenemy has a unique ability to use scheme cards, which involve a special mechanic related to their gameplay. When a scheme card is set in motion, it triggers its effects and can significantly impact the game state.

In an Archenemy setup, the Archenemy has the ability to do this at certain times during the game, and the rules governing these cards provide clear instructions for when and how they can be utilized to create an advantage over the other players. It's this specific context that makes option C the appropriate choice for understanding how a scheme card is activated.

The other options don't align with the specified conditions. They either overgeneralize the concept or misapply the unique mechanics that are particular only to Archenemy games. Understanding that scheme cards have specific rules around them during these special formats is crucial for grasping how they operate within the larger framework of Magic: The Gathering.