Quicksort's worst-case time complexity is ?(?2)O(n2), occurring when the pivot selection consistently results in unbalanced partitions, leading to many recursive calls and inefficient sorting, especially for sorted or nearly sorted data.

DNS (Domain Name System) primarily operates over UDP (User Datagram Protocol) for its query and response mechanism due to its low overhead and efficiency in real-time communication.

Adaptive Modulation refers to the technique of dynamically adjusting transmission parameters, such as power levels, to optimize performance and maintain stable connections in wireless networks.

Option 1, O(1), represents constant time complexity. Accessing an element in an array by its index takes the same amount of time, regardless of the array size. Hence, it is independent of the array's size, resulting in constant time complexity.

Roaming enables devices to switch between access points without losing connectivity, crucial for maintaining seamless connections as mobile devices move between different areas or networks.

Waterfall methodology follows a sequential approach where each phase must be completed before moving to the next, making it less adaptable to change. In contrast, Agile methodology promotes iterative development, continuous feedback, and changes in requirements, enabling better adaptability and responsiveness to change.

Decomposition is a critical step in dynamic programming where a complex problem is broken down into smaller, more manageable subproblems. This allows for the application of dynamic programming techniques, such as memoization or tabulation, to solve the problem efficiently.

SQL injection is a type of cyber attack where malicious SQL code is inserted into input fields of a web application, allowing attackers to access and manipulate the database. Using prepared statements or parameterized queries is an effective defense against SQL injection because they allow the database to distinguish between code and data, preventing attackers from executing arbitrary SQL commands. Sanitizing input and using escaped input are also good practices but may not provide as robust protection as prepared statements.

Dynamic programming can be broadly categorized into bottom-up (iterative) and top-down (recursive) approaches. Bottom-up DP starts from solving the smallest subproblems and builds up to the main problem, while top-down DP starts from the main problem and recursively solves smaller subproblems.

The SQL clause used to filter records in a SELECT statement is the WHERE clause. It allows you to specify conditions that the rows must meet to be included in the result set. This is essential for retrieving specific data based on criteria such as values in certain columns or combinations of conditions.