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.

The SQL SELECT statement is used to retrieve data from a database. It allows you to specify which columns you want to retrieve and which table(s) you want to retrieve the data from. Additionally, you can use various clauses like WHERE, ORDER BY, and GROUP BY to filter, sort, and group the retrieved data. In database testing, SELECT statements are commonly used to verify the correctness of data by querying specific tables and comparing the expected results with the actual results obtained from the database.

A B-Tree index is suitable for range queries, like searching products based on price, category, or brand, providing efficient retrieval of data based on range conditions. It organizes data in a hierarchical structure, making it ideal for search operations on large datasets.

Galois/Counter Mode (GCM) ensures that the same plaintext input will always result in different ciphertext outputs by combining the Counter (CTR) mode with Galois field multiplication. This mode provides high performance and parallelizability while maintaining the security of encryption. It's commonly used in applications where data integrity and confidentiality are paramount, such as in database encryption to prevent patterns from being discerned from repeated plaintexts.

The SQL AND clause is used to filter the result set based on multiple conditions. It allows you to combine two or more conditions in a SQL statement, ensuring that records meet all specified criteria simultaneously. This is particularly useful when you need to retrieve data that satisfies multiple conditions simultaneously, narrowing down the result set to only the desired records.

Multi-factor authentication (MFA) strengthens access control by requiring users to provide multiple forms of verification before granting access. Hence, the correct option is "Multiple." Single indicates only one form of verification, Complex and Unique do not accurately represent the requirement of multiple forms of verification.