What is a player not allowed to sacrifice?

The correct choice is based on the rules governing the concept of sacrificing permanents in Magic: The Gathering. A player can only sacrifice permanents they control. This means that if a permanent is not under their control, they lack the authority to sacrifice it, since the sacrifice is a game action that requires ownership and control of the object being sacrificed.

In the context of the options provided, sacrificing a permanent that a player does not control is simply not allowed. The rules clearly state that a player may only sacrifice their own permanents, which means choice B accurately reflects this limitation in gameplay.

The other options present scenarios that do not violate the rules regarding sacrifice. For instance, players can sacrifice their own permanents (such as creatures, artifacts, etc.), or even cards from their hand when instructed to do so by a game effect. Additionally, cards from the graveyard may be involved in other game actions, but are not eligible for sacrifice unless a specific effect allows it. Cards that have been revealed do not inherently lose their status as accessible game objects, but they too can only be sacrificed if they are under a player's control.