Which of the following rewards is gained through quest completion that impacts gameplay the most?

Gaining experience points through quest completion has a significant impact on gameplay because it allows players to level up their characters. Levelling up typically unlocks new skills, abilities, and sometimes access to advanced game mechanics, greatly enhancing a player's effectiveness in various game scenarios.

With increased levels, players can also encounter more challenging content, as their character's stats and capabilities improve. This progression is crucial in games where character development plays a prominent role, as it directly correlates with the player's ability to succeed in tougher encounters or explore new areas of the game.

While cards can expand a player's strategic options, achievements add a layer of challenge and recognition, and resources are crucial for crafting or upgrades, experience points are fundamentally tied to character growth and capability in a way that can transform the overall gameplay experience. Thus, the acquisition of experience points stands out as the reward that most fundamentally alters how a player interacts with the game world.