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 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.

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.

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.

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.

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.

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.

STP is vital for ensuring network stability and preventing broadcast storms caused by looping paths in Ethernet networks. By intelligently blocking redundant links while maintaining alternative paths, STP maintains a loop-free topology, optimizing network performance and reliability.

The correct option is "Connectivity." In distributed database systems, maintaining connectivity among different nodes and ensuring uninterrupted communication is a significant challenge. Network failures, latency issues, and bandwidth constraints can all impact the connectivity of distributed databases, affecting data access and consistency across the system. Overcoming these challenges requires robust networking protocols and fault-tolerant architectures.

Memoization involves storing the results of expensive function calls and returning the cached result when the same inputs occur again. Tabulation, on the other hand, builds the table from the ground up, typically using iterative loops. This is a top-down approach, while tabulation is bottom-up, starting from the smallest subproblems. Memoization is often easier to implement but can be less efficient for some problems compared to tabulation.