A clustered index dictates the physical order of data rows in the table, often based on the primary key column. In contrast, a non-clustered index stores a separate list of pointers to the actual data rows, allowing for different orders of retrieval. Understanding this distinction is crucial for efficient data retrieval and modification strategies.

The step of query optimization that involves choosing the most efficient execution plan for a SQL query is Optimization. During this phase, the database optimizer analyzes the query and generates alternative execution plans based on factors such as available indexes, table statistics, and system resources. The goal is to select the execution plan that minimizes the query's resource consumption and maximizes its performance. Once the optimal plan is determined, it is executed by the database engine.

"Data at rest" encryption refers to the practice of encrypting data while it's stored on disk. This ensures that even if someone gains unauthorized access to the physical storage medium, they won't be able to read the data without proper decryption keys. It's a crucial aspect of database security, protecting sensitive information from unauthorized access at rest. This encryption typically involves using algorithms and keys to convert plaintext data into ciphertext, rendering it unreadable without the appropriate decryption process. Hence, option 2 is correct.

Test data generation tools automate the creation of test data, which helps in ensuring that databases are tested thoroughly and efficiently. By automating this process, testers can save time and reduce the likelihood of human errors.

The phase of data extraction from the source system is typically the initial step in the data migration process, where data is fetched for further processing.

A LEFT JOIN is used when you want to retrieve all records from the left table, regardless of whether there are matching records in the right table. It includes all records from the left table and the matching records from the right table. This is useful when you want to retrieve data from one table even if there are no corresponding records in the other table.

Database testing tools automate the creation of test scripts for various scenarios, enabling testers to execute tests efficiently and consistently. Test scripts encompass a series of actions to be performed during testing, making option 3, "Script," the correct choice.

The primary key in SQL uniquely identifies each row in a table. It ensures that each row is distinct from others and serves as a reference point for other tables when establishing relationships.

Symmetric encryption transforms data into an unreadable format using a single encryption key. This encrypted data can only be decrypted using the same key, ensuring confidentiality. Without the decryption key, it is virtually impossible to revert the data to its original form, enhancing security. This method is commonly used for securing data in databases.

Without proper database testing, data anomalies can go undetected. These anomalies could include inconsistencies, duplicates, or incorrect data entries, leading to data quality issues and potentially impacting business decisions and operations.