Memory compaction is a technique used in memory management to reduce fragmentation by rearranging memory contents to place all free memory together in contiguous blocks. This helps in maximizing the available memory for allocating new processes or data structures, especially in systems where memory fragmentation can lead to inefficient memory usage.

The correct option is "Hypervisor." The hypervisor layer is responsible for abstracting physical hardware resources such as CPU, memory, and storage, allowing multiple virtual machines (VMs) to share the underlying physical resources efficiently. This abstraction enables better resource utilization and flexibility in managing virtualized environments.

The text-decoration property in CSS is used to control the appearance of text decorations such as underline, overline, line-through, and blink. This property is commonly used to style links and text decorations.

Binary search is typically performed on a sorted array. The reason for this is that binary search requires the elements to be in sorted order to efficiently divide and conquer the search space, reducing the time complexity to O(log n) compared to O(n) for linear search in an unsorted array.

Fourth Normal Form (4NF) is the normal form that allows multi-valued dependencies to be removed. 4NF is an extension of third normal form (3NF) and deals specifically with situations where there are multiple independent multi-valued dependencies between attributes. By decomposing the table into smaller tables and ensuring each table has a single theme, 4NF helps in removing such complex dependencies and reducing data redundancy.

The correct SQL keyword used to retrieve data from a database is SELECT. This keyword is followed by specific columns or a wildcard (*) to indicate all columns that should be retrieved from a table or tables in the database. Using SELECT is fundamental in SQL as it forms the basis of querying data from databases.

Distributed file systems use multiple servers to store and manage files, offering scalability and fault tolerance. In contrast, traditional file systems often rely on a single server, which can become a bottleneck and a single point of failure. This fundamental difference impacts scalability, fault tolerance, and overall system resilience.

The creation of threads is typically less costly in terms of system resources compared to creating processes. Threads share memory space within a process, while processes have separate memory spaces. Semaphores, monitors, and pipelines are synchronization mechanisms used in concurrent programming but are not directly related to the cost of creation in terms of system resources.

Dynamic programming is commonly used for resource optimization problems, where the goal is to optimize the allocation of resources such as time, money, or personnel to achieve the best possible outcome. This technique involves breaking down the optimization problem into smaller subproblems and using optimal substructure to find the overall optimal solution.

Option 1 suggests using parameterized queries or prepared statements, which are fundamental to preventing SQL injection attacks by separating user input from SQL commands. Option 3 involves additional security measures like WAF and database permissions, which are beneficial but secondary to fixing the core vulnerability. Option 4 addresses secure coding practices but does not focus specifically on remedying SQL injection vulnerabilities.

Shortest Job First (SJF) scheduling algorithm selects the process with the shortest burst time to execute next. This minimizes the average waiting time and improves system throughput by prioritizing smaller jobs. SJF can be either preemptive or non-preemptive, where in preemptive SJF, a new job with a shorter burst time can interrupt the currently running job.

A file system in an operating system primarily serves to organize and manage files and directories efficiently, enabling users and applications to store, retrieve, and manipulate data effectively. This organization includes managing file metadata such as permissions, timestamps, and file locations. The file system also ensures data integrity and reliability through mechanisms like journaling and redundancy. Overall, it provides a structured approach to data storage and retrieval within an operating system.