「日本の電車は時間に正確ですよ」「___、だから皆さん時計みたいに使うんですね」
Q. Which response shows understanding ('I see, that explains it')?
Translation
"Japanese trains are punctual." "I see — that's why people use them like clocks."
Explanation
なるほど signals that new information makes sense to you. そろそろ ('about time to') and わざわざ ('going out of one's way') don't function as responses of understanding.