What occurs immediately after combat damage has been assigned during the combat damage step?

In the combat damage step of a game of Magic: The Gathering, after combat damage has been assigned, the next action that occurs is that combat damage is dealt simultaneously. This means that all creatures involved in combat will deal their assigned damage to the opposing creatures or players at the exact same time.

This simultaneous application of damage is crucial to understanding combat interactions, as it allows for various game mechanics to function properly, such as damage prevention effects or creatures with abilities that might trigger upon dealing or receiving damage. By dealing damage simultaneously, players cannot respond to damage being dealt from one source before the others, maintaining the integrity of combat interactions.

The other choices reflect actions that occur either before combat damage is assigned or not relevant to the immediate effects following the assignment of combat damage. For instance, creature declaration and activation of abilities must occur in earlier steps of the combat phase, while managing mana costs is more related to casting spells rather than resolving combat.