If a player in a sealed deck tournament adds cards to his card pool, what infraction is this?

The scenario describes a player adding cards to their sealed deck card pool, which is an unauthorized alteration of the materials they are allowed to use in the tournament. This action falls under the definition of Manipulation of Game Materials, which specifically addresses improper handling or adjustment of game components by a player.

When a player alters their card pool beyond the rules of the sealed deck format, it creates an unfair advantage and disrupts the integrity of the competition. This infraction is serious because it can impact game outcomes and the overall fairness of the tournament.

In contrast, the other options pertain to different types of infractions. Illegal Deck Construction relates to situations where a player's deck does not conform to the allowed card pool before the tournament, Hidden Information Violations involve situations where a player fails to reveal information that should be visible, and Collusion pertains to players working together to gain an unfair advantage, which is not applicable in this case of modifying card pools.