The correct answer is Transaction isolation level settings.

Transaction isolation level settings determine how concurrent transactions see and interact with each other and they directly control the isolation guarantee of ACID. By selecting levels such as read uncommitted read committed repeatable read or serializable the database prevents undesirable phenomena like dirty reads non repeatable reads and phantom reads and thus enforces isolation among transactions.

Transaction isolation level settings operate together with atomic commit protocols and logging to provide full ACID behavior but the isolation configuration is the database property that specifically governs how strictly transactions are isolated from one another.

Database normalization is a design technique for organizing schema to reduce redundancy and update anomalies. It does not control transactional concurrency or isolation and therefore it does not enforce ACID transaction semantics.

Transaction logging and write ahead logs are essential for durability and for recovering to a consistent state after a crash and they support atomic commits. They do not by themselves control how concurrent transactions are isolated so they are not the property that enforces isolation across transactions.

High availability and failover focus on keeping the database available during failures and on minimizing downtime. These features help with availability and continuity but they do not directly enforce the transactional ACID guarantees such as isolation.

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