This question's explanation and your answers so far.
データベースの障害に備えて, 毎日業務終了後にバックアップファイルを取得している。毎週土曜日にはフルバックアップファイルを取得し, 日曜日から金曜日は差分バックアップファイルを取得する。差分バックアップファイルとは, 最後のフルバックアップファイルから, 差分バックアップファイルを取得する当日までの, 全ての差分データのバックアップファイルである。水曜日にデータベースが破損したので, 前日の火曜日の業務終了時点のデータベースに復旧することにした。このとき, データベースを復旧させるために最低限必要なバックアップファイルはどれか。
With only differential files and no Saturday full backup to serve as the baseline, the foundation of the database cannot be restored at all, so recovery is impossible. In addition, since differential backups are cumulative, the Sunday and Monday differentials are already contained in the Tuesday differential and are unnecessary, so this is not the minimum combination either.
The Tuesday differential is a file that records only the changed portions, and on its own there is no foundation to restore from. Because the Saturday full backup that serves as the baseline is missing, recovery cannot be done.
In addition to the full backup, it applies all of the Sunday, Monday, and Tuesday differentials, so recovery itself is possible; however, with cumulative differential backups the Tuesday differential already contains all of the Sunday and Monday changes, so the Sunday and Monday differentials are redundant and the requirement of being the minimum is not met. This is actually the procedure for an incremental backup scheme, and does not apply to the differential scheme in this question.
正解 A differential backup uses a cumulative approach: each time, it records all of the differential data from the last full backup up to that day as a single set. Therefore the Tuesday differential backup alone already contains every change made after the Saturday full backup up to the end of business on Tuesday. The restore procedure is: first use the Saturday full backup to return the database to its Saturday state, then apply the Tuesday differential backup once on top of it, and the state at the end of business on Tuesday is reproduced. Thus the minimum required is just the two files, the Saturday full backup plus the Tuesday differential backup, so エ is correct.