The purpose of the Singleton design pattern is to ensure that a class has only one instance and provide a global point of access to it. This is particularly useful in scenarios where you want to control access to resources or manage shared resources efficiently. The pattern also helps in maintaining a single instance throughout the application's lifecycle, preventing unnecessary duplication and ensuring consistency in data and behavior.

Firewalls play a crucial role in web application security by filtering network traffic to identify and block malicious or unauthorized requests. They act as a barrier between the internet and the web server, inspecting incoming and outgoing data packets based on predefined rulesets. By analyzing factors such as IP addresses, port numbers, and packet contents, firewalls can detect and thwart various cyber threats, including DDoS attacks, SQL injection attempts, and unauthorized access attempts. This proactive defense mechanism helps prevent malicious entities from compromising the web application's integrity, ensuring continuous availability and data confidentiality for users.

The correct option is 'Top.' To implement a stack using arrays, we use a 'Top' pointer to keep track of the topmost element in the stack, allowing us to perform push and pop operations efficiently.

The correct option is "AVG()." The AVG() function in SQL is used to calculate the average value of a numeric column in a table. It excludes NULL values while calculating the average.

Paging is a memory management scheme where physical memory is divided into fixed-sized blocks called frames, and logical memory is divided into fixed-sized blocks called pages. Each process is then assigned contiguous blocks of frames for its pages. Paging helps in efficient memory utilization and reduces external fragmentation.

The purpose of the GROUP BY clause in SQL queries is to aggregate data based on specified columns. It is used in conjunction with aggregate functions such as COUNT, SUM, AVG, etc., to group rows that have the same values in specified columns. This allows for summarizing data and performing calculations on grouped data sets.

The File Allocation Table (FAT) is a crucial component in a file system that maintains a record of which disk blocks are free and which are allocated to files. It's commonly used in older file systems like FAT32.

A race condition occurs in concurrent systems when the outcome of operations depends on the timing or interleaving of multiple threads. It leads to data inconsistency and unpredictable results. Synchronization mechanisms like mutexes and semaphores are used to prevent race conditions by ensuring that only one thread can access a shared resource at a time, thereby maintaining data integrity and thread safety.

A segmentation fault occurs when a program attempts to access a memory address that is not within its allocated memory space. This can happen due to bugs in the program's code, such as accessing an uninitialized pointer or trying to write to read-only memory. Insufficient memory space or buffer overflows can indirectly lead to segmentation faults by causing memory corruption.

Shortest Job First (SJF) scheduling minimizes average waiting time by prioritizing shorter jobs, which reduces the overall waiting time for processes in the ready queue. This approach is effective in scenarios where smaller tasks can be completed quickly, leading to better system responsiveness and improved resource utilization. However, SJF may face challenges in predicting job lengths accurately, especially in dynamic workloads, requiring adaptations like Shortest Remaining Time First (SRTF) to handle varying job durations effectively.