Who can activate an activated ability in gameplay?

The correct choice emphasizes that an activated ability can only be activated by the controller of that ability. The controller is typically the player who has control over the object—this is often the player who played the card or who has it under their control due to game mechanics. If a permanent, such as a creature with an activated ability, has no controller (for instance, if it was somehow removed from the game or if the game state lacks a rightful controller), its owner may activate its abilities as long as they can pay the associated costs.

This is important because it establishes clear rules on who can interact with a permanent's activated abilities, which is crucial for maintaining the integrity of gameplay.

While other choices suggest broader permissions for activation—like allowing any player at the table or even any player during their turn to activate abilities—these do not accurately reflect the rules. Only the specific controller or, in unique situations, the owner of an object can activate its abilities, making that the correct understanding of the mechanics involved.