This question's explanation and your answers so far.
毎週日曜日の業務終了後にフルバックアップファイルを取得し, 月曜日～土曜日の業務終了後には増分バックアップファイルを取得しているシステムがある。水曜日の業務中に故障が発生したので, バックアップファイルを使って火曜日の業務終了時点の状態にデータを復元することにした。データ復元に必要なバックアップファイルを全て挙げたものはどれか。ここで, 増分バックアップファイルとは, 前回のバックアップファイル (フルバックアップファイル又は増分バックアップファイル) の取得以降に変更されたデータだけのバックアップファイルを意味する。
正解 An incremental backup file stores only the data changed since the previous backup. Therefore, to restore to the state at the end of business on Tuesday, you must first restore all data using Sunday's full backup as the base point, then apply on top of it Monday's incremental (the changes since Sunday) and next Tuesday's incremental (the changes since Monday) in order. Only by using all three of these files can the complete state at the end of business on Tuesday be reproduced, so option ア, which lists “Sunday's full backup file, and Monday's and Tuesday's incremental backup files”, is correct. Note that Wednesday's failure occurred during business, so the end-of-business incremental backup had not yet been taken, and it is not needed for restoring to the Tuesday point anyway.
This has only Sunday's full backup and Tuesday's incremental, and is missing Monday's incremental. Tuesday's incremental holds only the changes since Monday's backup, so if you apply only Tuesday's incremental without applying Monday's incremental, the data changed on Monday is not restored and the state becomes incomplete.
This has only Monday's and Tuesday's incrementals, without Sunday's full backup as the base point. An incremental contains only the changed data, so unless you start from a full backup that holds all the data, the entire original data cannot be rebuilt and restoration is impossible.
With only Tuesday's incremental, both the base full backup and Monday's incremental are missing, so only part of the changed data is obtained. The base point and the intermediate change history needed to reproduce all the data are not complete, so the state at the end of business on Tuesday cannot be restored.