A subquery in SQL is a query nested within another query, often used within the WHERE or HAVING clause. It is different from a regular query as it is embedded within another query and helps in retrieving data based on certain conditions or criteria.

Steganography involves concealing the existence of a message, making it invisible to unintended recipients, whereas encryption involves transforming the message into a secure format but doesn't hide its existence.

The Observer design pattern is the most suitable for notifying different components about changes in a specific object. This pattern establishes a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. This ensures loose coupling between the subject (the object being observed) and its observers, promoting flexibility and maintainability in the system.

The primary function of the Transport layer in the TCP/IP model is flow control. This involves managing the rate of data transmission between source and destination to prevent congestion and ensure efficient delivery.

RADIUS (Remote Authentication Dial-In User Service) is a protocol used for centralizing authentication, authorization, and accounting management in a network, commonly used in dial-up scenarios and for Wi-Fi networks.

Page replacement algorithms play a crucial role in memory management by deciding which pages should be swapped out of main memory when the memory is full and a new page needs to be brought in. Examples of page replacement algorithms include FIFO (First-In-First-Out), LRU (Least Recently Used), and Optimal algorithms. FIFO replaces the oldest page, LRU replaces the least recently used page, and Optimal replaces the page that will not be used for the longest time in the future. These algorithms aim to minimize page faults and improve overall system performance.

Microservices architecture involves developing applications as a collection of small, independent services. Containerization technologies such as Docker facilitate the deployment and management of these microservices by encapsulating each service in a container, providing scalability and portability benefits.

The correct option is "INSERT INTO." The INSERT INTO statement in SQL is used to add a new record or row to an existing table. It allows you to specify the columns and their corresponding values for the new record.

Unvalidated input is a security vulnerability where an application does not properly validate or sanitize user input before processing it. This can lead to various attacks such as SQL injection, cross-site scripting (XSS), and command injection. Proper input validation and sanitization are essential to prevent such vulnerabilities and protect the application from malicious exploitation.

In JavaScript, variables can be declared using the var, let, or const keywords. Option 2 shows the correct syntax using the let keyword, which is commonly used for variable declaration in modern JavaScript.