In Oracle Database, which combination correctly lists the standard transaction isolation levels that are officially supported for transactions?

Introduction / Context:
In relational database systems, the transaction isolation level controls how concurrent transactions observe each other's changes. Oracle Database is widely used in enterprise applications, and understanding which isolation levels it supports is essential for tuning consistency and performance. Many interview questions try to check whether you know that Oracle does not implement every isolation level described in the SQL standard, but focuses on a smaller, well defined set.

Given Data / Assumptions:

• The database platform in question is Oracle Database.
• We are talking about transaction isolation levels, not locking modes or read only cursors.
• The question asks which levels are officially supported for transactions.
• We consider the typical, documented behaviour of modern Oracle releases.

Concept / Approach:
The SQL standard describes four isolation levels: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE. Oracle does not implement READ UNCOMMITTED; dirty reads are never allowed. Oracle also does not provide REPEATABLE READ as a separate isolation level. Instead, Oracle offers READ COMMITTED as the default isolation level and SERIALIZABLE as a stronger level that provides serializable transaction behaviour. Oracle also supports read only transactions, but read only is usually treated as a mode rather than a separate isolation level in exam style questions. Therefore the correct answer should list READ COMMITTED and SERIALIZABLE as the supported isolation levels.

Step-by-Step Solution:
Step 1: Recall the four SQL standard isolation levels: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE. Step 2: Remember that Oracle never allows dirty reads, so READ UNCOMMITTED is not supported as a separate isolation level. Step 3: Note that Oracle uses a multiversion read consistency model and does not expose REPEATABLE READ as an explicit isolation level for transactions. Step 4: Recognise that Oracle documentation describes READ COMMITTED as the default level and SERIALIZABLE as an alternative level that gives serializable behaviour. Step 5: Conclude that the pair READ COMMITTED and SERIALIZABLE is the correct combination of supported isolation levels for Oracle transactions.

Verification / Alternative check:
You can verify this understanding by recalling typical Oracle commands. When you issue SET TRANSACTION ISOLATION LEVEL READ COMMITTED or SET TRANSACTION ISOLATION LEVEL SERIALIZABLE, Oracle accepts these statements. However, attempts to set isolation levels such as READ UNCOMMITTED or REPEATABLE READ are not supported and will not behave as separate transaction modes. This behaviour confirms that only READ COMMITTED and SERIALIZABLE are the conventional transaction isolation levels in Oracle exams and documentation, with read only treated as a specialised transaction option rather than a general level.

Why Other Options Are Wrong:
Option READ UNCOMMITTED and READ COMMITTED: Oracle does not allow READ UNCOMMITTED dirty reads, so this combination is incorrect. Option REPEATABLE READ and SNAPSHOT: Oracle does not expose REPEATABLE READ as a standard isolation level, and SNAPSHOT is not an Oracle transaction isolation keyword in this form. Option READ COMMITTED, REPEATABLE READ, SERIALIZABLE, READ UNCOMMITTED: This list describes the full set of SQL standard levels, but Oracle does not implement all of them as separate modes.

Common Pitfalls:
A common mistake is to memorise the SQL standard isolation levels and assume that every database supports all four. Another pitfall is to confuse Oracle's multiversion read consistency, which can make some nonrepeatable read phenomena less visible, with the existence of a REPEATABLE READ isolation level. Students also sometimes think that read only is a separate isolation level instead of a transaction property. For exam purposes, it is important to remember that Oracle focuses on READ COMMITTED and SERIALIZABLE as its primary isolation levels.

Final Answer:
Oracle Database supports the transaction isolation levels READ COMMITTED and SERIALIZABLE.