Who owns a token created by a card effect that instructs an opponent to create it?

The correct answer is that the token is owned by the controller of the effect. When a card effect instructs an opponent to create a token, it is typically a result of the card's controller exercising their abilities in the game. Even though the opponent is the one creating it, it is the controller of the card who has the ownership rights over the token generated by that effect.

This reflects the game's mechanics regarding ownership and control, which often dictate that the player whose card's effect is being resolved takes charge of the outcome, including any tokens that the effect generates. Thus, the tokens generated from such an effect remain tied to the controller of the card that initiated the action.