If a creature's toughness is greater than 0, but it has been dealt lethal damage, what happens next?

When a creature has been dealt lethal damage, it is subject to the rules regarding state-based actions in Magic: The Gathering. Lethal damage is defined as damage equal to or greater than the creature's toughness.

In this scenario, if the creature's toughness is greater than 0 and it has been assigned damage equal to that toughness or more, it is considered to have lethal damage. According to the comprehensive rules, this triggers a state-based action that results in the creature being destroyed.

Once the conditions for destruction are met—specifically that a creature has lethal damage and has a toughness greater than 0—the game checks for state-based actions and removes the creature from the battlefield immediately, placing it into its owner's graveyard. This process ensures that the game maintains order and that damaged creatures do not remain in play where they are no longer functional.

Thus, the correct answer accurately reflects the game mechanics surrounding damage and the state of creatures in play.