Encapsulation is one of the main principles of Object-Oriented Programming (OOP). It refers to the bundling of data (attributes) and methods (functions) that operate on the data into a single unit or class. Encapsulation helps in hiding the internal complexities of an object and allows access to only the necessary parts through well-defined interfaces. This concept promotes modularity, reusability, and easier maintenance of code in OOP.

A clustered index stores a copy of the indexed columns along with the index data structure itself. This arrangement of data storage directly corresponds to the order of the indexed columns, making retrieval of data faster when queries align with the index order. However, it's important to note that a table can only have one clustered index, typically on the primary key column.

Hashing is used to ensure data integrity by generating a fixed-size hash value for a given input data. This hash value can be compared to verify data integrity during transmission or storage.

802.11a operates in the 5 GHz band, providing faster data rates but shorter range. 802.11b operates in the 2.4 GHz band with slower speeds but better range. 802.11g combines elements of both for improved speed and range.

Regression testing involves re-running test cases to ensure that new changes haven't adversely affected existing functionalities. It verifies that the code behavior remains unchanged post-modifications. This is crucial in software development to catch bugs introduced inadvertently during updates and to maintain overall system reliability and stability. It also helps in ensuring that new features don't break existing functionalities.

The Adapter design pattern allows objects with incompatible interfaces to work together. It acts as a bridge between the client and the target class, enabling communication and interaction. This pattern is useful when integrating existing systems that have incompatible interfaces or when designing reusable libraries that need to work with multiple interfaces.

Graph databases are ideal for handling complex relationships as they excel in managing interconnected data. Nodes represent entities like users, and edges represent relationships between them. This structure is well-suited for social networks where users have varied connections. Document databases are better suited for semi-structured data and may not handle complex relationships as effectively as graph databases. Columnar databases are optimized for analytical queries, not for complex relationship handling. Key-value databases are efficient for simple data retrieval but lack the structure needed for complex relationships.

SQL injection attacks occur when malicious code is inserted into SQL statements, exploiting vulnerabilities to manipulate the database. To prevent SQL injection, input data should be sanitized to remove any malicious code. Using stored procedures and encrypting database connections also enhance security but may not directly prevent SQL injection attacks.

The correct option is Design. In the design phase of the SDLC, the software system's architecture, structure, modules, interfaces, and data flow are planned and documented. This phase bridges the gap between the requirements gathered and the actual implementation. It involves creating detailed technical specifications, such as system diagrams, database designs, user interface designs, and algorithmic designs. The design phase lays the groundwork for the development team to start coding based on the established design.

NoSQL databases commonly use JSON (JavaScript Object Notation) as a format for storing and retrieving data. JSON is particularly well-suited for NoSQL databases due to its flexibility, simplicity, and ease of integration with various programming languages and frameworks, making it a popular choice for developers working with these databases.