Final answer:
The correct answer is option b. serializable. This isolation level ensures that only committed data can be read by a transaction and prevents other transactions from updating the data between reads.
Step-by-step explanation:
The correct answer is option b. serializable. The serializable isolation level allows only committed data to be read by a transaction and ensures that no other transaction can update the data item between two reads. This level guarantees a high level of consistency.
For example, let's say we have two transactions: T1 and T2. T1 reads a data item, and before it can perform a second read, T2 updates the data item. In the serializable isolation level, T1 would not be allowed to read the updated data item. This ensures that T1 always sees a consistent view of the data.
Other isolation levels, such as read uncommitted, repeatable read, and read committed, do not provide the same level of data consistency and may allow for dirty reads, non-repeatable reads, or phantom reads.
Once a record is read, it is locked until the transaction is complete, preventing other transactions from writing to it and ensuring that subsequent reads will see the same data. The repeatable read isolation is one level below the strictest level, which is serializable, and one level above read committed, which only prevents dirty reads but allows non-repeatable reads. It is an important concept in database management systems to maintain data integrity and provide a stable view of data to a transaction.