① Fn.txt is a file held by the current directory (the DIRB marked ＊) itself. To reference it, “Fn.txt” (or “．￥Fn.txt”) would suffice, which does not match this question's specification that goes up 2 levels with “．．”.
×Incorrectイ
② Fn.txt is a file directly under the upper DIRB, two levels up. “．．￥．．” does reach this upper DIRB, but the specification then descends into a child directory with “DIRB”, so the target is not ② directly under DIRB but the file reached via the child DIRB.
×Incorrectウ
③ Fn.txt is a file inside DIRB, which is a further child of the lower DIRB. To reach it, you would need to follow DIRB twice, as in “．．￥．．￥DIRB￥DIRB￥Fn.txt”; since this specification has DIRB only once, it stops directly under the lower DIRB (at ④).
○エ
正解The current directory (marked ＊) is the innermost DIRB reached by following Root → upper DIRB → DIRA → DIRB (it holds ① Fn.txt). Resolve the specified path “．．￥．．￥DIRB￥Fn.txt” from left to right. First, “．．” moves one level up (to the parent DIRA); the next “．．” moves one more level up (back to the upper DIRB, directly under the root). This upper DIRB has three children: DIRA, ② Fn.txt, and another DIRB, and “DIRB” refers to this lower DIRB. The final “Fn.txt” is the file directly under that lower DIRB, namely ④ Fn.txt. Therefore the file referenced is ④, and the correct answer is エ.
ここがポイント
In relative path specification, “．．” denotes one level up (the parent directory) and “．” denotes the current directory. Starting from the current directory, you go up to a parent with “．．” and descend into a child by directory name, tracing the path segment by segment.
In a hierarchy that contains multiple directories with the same name (DIRB), the key is to correctly identify which DIRB at which level is meant, based on the parent-child relationships.