When can players view their opponent's graveyard during a match?

Players can view their opponent's graveyard anytime during the game as part of the general principles of Magic: The Gathering regarding information access. The game's rules state that a player is allowed to look at their opponent's graveyard at any point, which helps maintain strategic play and enables players to make informed decisions based on what has been played and is available. This openness is crucial for the competitive environment, as it allows players to consider their opponent's potential plays and the state of the game.

The other options do not align with the rules and principles of Magic: The Gathering. Limiting viewing to only one's own turn or only at the end of the match would hinder strategic depth and the ability to assess the game state effectively. Additionally, conditional access to the graveyard based on an opponent's permission would contradict the fundamental accessibility of information in the game. Consequently, the ability to view the graveyard at any time is essential for informed decision-making and overall fair play in matches.