※ Our check while writing this explanation could not fully confirm the published answer; scoring follows IPA's official key.
×Incorrectア
Because the file name begins with “related_”, sorting by file name makes the related materials clump together with the other files that begin with “related_”, displayed at a position far from the business reports that begin with “report_”. This fails the very requirement that "related materials are displayed next to the business report." In addition, the sequential number is a single-digit n, so it cannot number up to 12 related materials either.
×Incorrectイ
The sequential number is a two-digit nn, so 12 items can be numbered, but just as with ア the file name begins with “related_”. After sorting, the related materials alone gather in a separate place and do not line up next to the corresponding business report, so the ordering requirement is not satisfied.
×Incorrectウ
Its beginning is the same “report_”+ yymm + w + author name as the business report, so it satisfies the ordering requirement. However, the sequential number is fixed at a single digit n, so only 9 numbers, from 1 to 9, can be assigned. Since up to 12 related materials can be kept per business report (creation rule (5)), there is no number left to assign from the 10th item on, and the count requirement cannot be met. A two-digit nn that can express up to 12 is required.
○エ
正解The business report file name defined in Table 1 is “report_”+ yymm + w + author name. To make a related material appear next to its corresponding business report when files are sorted by file name, the file name of the related material has no choice but to begin with exactly the same string as the business report, with the distinguishing information attached after it. エ begins with “report_”+ yymm + w + author name and attaches “_related_”+ nn at the end, so after sorting, only the related materials of that report line up together immediately after the business report. Furthermore, creation rule (5) allows up to 12 related materials to be kept per business report, so the sequential number must be able to express up to 12. Since nn has two digits, all 12 items from 01 to 12 can be numbered, and because the digit count is fixed (zero-padded), sorting them as strings still gives 01, 02, …, 12 — the same order as the sequential numbers. エ is the only option that satisfies both the ordering requirement and the count requirement.
ここがポイント
When you want related files to sit next to each other under file-name ordering, make the "beginning part" of the names common across the group of files you want to keep together, and put the information that distinguishes them (a sequential number and so on) at the end. Sorting compares characters in order starting from the first one, so if the beginning differs the file jumps off to a different place.
For a sequential number, secure "enough digits to express the required number of items," and also fix the digit count and pad with zeros. Here the upper limit is 12 items, so two digits (01–12) are needed, and by aligning the digit count you can make the ordering as strings agree with the ordering by numeric size.