This question's explanation and your answers so far.
次のアローダイアグラムに基づき作業を行った結果，作業 D が2日遅延し，作業 F が3日前倒しで完了した。作業全体の所要日数は予定と比べてどれくらい変化したか。 （アローダイアグラム。凡例は矢線の上に「作業名」，下に「所要日数」を表す） - 開始結合点 → 作業 A（所要日数 2）→ 結合点1 - 開始結合点 → 作業 B（所要日数 3）→ 結合点2 - 結合点1 → 作業 C（所要日数 4）→ 結合点3 - 結合点2 → 作業 D（所要日数 1）→ 結合点3 - 結合点2 → 作業 E（所要日数 1）→ 結合点4 - 結合点3 → 作業 F（所要日数 5）→ 終了結合点 - 結合点4 → 作業 G（所要日数 5）→ 終了結合点
The direction (sign) of the change is reversed. The total duration shrinks from 11 to 9 days (earlier), but choice ア treats it the opposite way, as a delay. The value of 3 days is also not the correct answer of 2 days; it borrows the 3-day amount by which activity F was moved up. This makes it a wrong-direction decoy that flips the sign of choice エ (3 days earlier) into a delay.
The direction is correct but the number of days is wrong. It hastily subtracts the 2-day delay of activity D from the 3-day advance of activity F (3 - 2 = 1) and concludes 1 day earlier. In reality the critical path must be recomputed after the change: the new longest path is B->E->G = 9 days, so the reduction is 11 -> 9 = 2 days, not 1 day.
正解 In an arrow diagram, the longest path from start to finish (the critical path) determines the total duration of the whole project. Initially the paths are A->C->F = 2+4+5 = 11 days, B->D->F = 3+1+5 = 9 days, and B->E->G = 3+1+5 = 9 days, so the longest is A->C->F at 11 days. After the change, activity D is delayed by 2 days (D = 1 -> 3 days) and activity F is finished 3 days early (F = 5 -> 2 days). Recomputing each path gives A->C->F = 2+4+2 = 8 days, B->D->F = 3+3+2 = 8 days, and B->E->G = 3+1+5 = 9 days, so the longest path shifts to B->E->G at 9 days. The total duration changes from 11 days to 9 days, which is 2 days less than planned, i.e. 2 days earlier. Therefore the correct answer is choice ウ.
This mistake looks only at activity F on the original critical path A->C->F = 11 days being moved up by 3 days, and hastily computes 11 - 3 = 8 days, i.e. 3 days earlier. Once F is shortened, A->C->F becomes 8 days and is no longer the longest; B->E->G = 9 days becomes the new critical path. So the total stays at 9 days (2 days earlier) and does not become 3 days earlier.