Which of the following actions constitutes sacrificing?

The action that constitutes sacrificing is specifically moving a permanent from the battlefield to the graveyard. In the context of Magic: The Gathering, sacrificing a permanent typically means that the player voluntarily chooses to send it to the graveyard, often as part of the cost of an ability or spell. This action is distinct in that it usually includes specific language such as "sacrifice a creature" in the instructions or rules of an effect, indicating an intentional decision by the player to remove that permanent from play.

The other actions listed do not align with the definition of sacrificing. For instance, dealing damage does not remove a permanent from the battlefield unless it causes lethal damage and the permanent is destroyed, which is a different mechanic. Activating a permanent's ability refers to using one of its effects, but it does not inherently involve sending the permanent to the graveyard. Revealing a card from a player's hand has no impact on the battlefield and does not involve any permanents whatsoever. Therefore, the clear and correct understanding of sacrificing is demonstrated by the act of moving a permanent from the battlefield to the graveyard.