Which SQL isolation level is the lowest one that prevents dirty reads, meaning reading data written by a transaction that has not yet committed?
Correct answer B. Read Committed
Explanation
The SQL standard defines four isolation levels by the phenomena they forbid. Read Uncommitted permits dirty reads, Read Committed is the first level that forbids them, Repeatable Read also forbids non-repeatable reads, and Serializable forbids all anomalies. Repeatable Read and Serializable prevent dirty reads too, but they are not the lowest. PostgreSQL never allows dirty reads even at Read Uncommitted.
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