In external sorting and sequential file organisation, which of the following methods are used when storing or merging sequential files on secondary storage?

Introduction / Context:
When data sets become too large to fit entirely into main memory, external sorting techniques are used. These methods rely on storing data in sequential files on secondary storage such as disks or tapes and then performing multiway merges of sorted runs. Concepts like natural merging, polyphase sorting, and distribution of initial runs are closely associated with external merge sort implementations. This question tests whether you can recognise that these techniques all belong to the family of methods used in the organisation and processing of sequential files.

Given Data / Assumptions:

• We are dealing with sequential file organisation and external sorting.
• Natural merging, polyphase sort, and distribution of initial runs are all mentioned as possible methods.
• The question asks which of these are used in the context of storing and processing sequential files.
• We assume standard textbook descriptions of external merge sort techniques.

Concept / Approach:
External merge sort typically proceeds in two phases. In the first phase, the input file is scanned, and sorted runs are created, either by reading chunks into memory and sorting them or by using replacement selection to produce longer runs. This stage is sometimes referred to as the distribution of initial runs, because these runs are written out to multiple work tapes or temporary files. In the second phase, these runs are repeatedly merged to produce longer and longer sorted runs until a final sorted file is obtained. Natural merging uses the natural ordering of the data to identify runs that are already sorted and merges them. Polyphase sorting is an optimised form of multiway merging that makes efficient use of multiple tapes or files by varying the number of runs on each device. All three techniques are standard tools in the design of sequential file processing systems.

Step-by-Step Solution:
Step 1: Recognise that distribution of initial runs is part of the first phase of external sorting, where sorted subfiles are created. Step 2: Recall that natural merging is a variant of merge sort that takes advantage of segments of the file that are already in sorted order and merges these natural runs. Step 3: Remember that polyphase sort is a multiway merging strategy that uses variable run distributions on several tapes or files to reduce the number of passes. Step 4: Note that each of these methods is directly related to the handling and processing of sequential files in external sorting scenarios. Step 5: Conclude that the best answer must include all three methods, which leads to the choice All of the above.

Verification / Alternative check:
If you review any standard text on file organisation and external sorting, you will see chapters that first discuss how to generate and distribute initial runs, then how to merge them with techniques like natural merging and polyphase merge. The fact that these techniques appear together and are all described as parts of external merge sort confirms that treating them as a unified group is correct. Therefore, it is reasonable that all three are considered methods available when storing and processing sequential files.

Why Other Options Are Wrong:
Option Natural merging: This is a valid method, but it is only one part of the full set of techniques and does not cover run distribution or optimised multiway merging on its own. Option Polyphase sort: Also a valid method, but again only one approach within the broad topic of external sorting. Option Distribution of initial runs: This describes only the first phase of external sorting and does not include the merging strategies themselves.

Common Pitfalls:
Students sometimes memorise one or two external sorting techniques and assume that other names refer to unrelated concepts. Another common mistake is to think that only merging is relevant and to overlook the crucial role of generating and distributing initial runs. Remember that real external sorting implementations are pipelines, not single steps, and names like natural merging, polyphase sort, and distribution of initial runs all point to different stages or optimisations within that pipeline.

Final Answer:
All of the listed methods are used in sequential file handling and external sorting, so the correct choice is All of the above.