Isn't the correct sentence given grammatically incorrect? 'Tôi ngay lập tức mở cuốn sách', shouldn't it be 'Tôi mở cuốn sách ngay lập tức' instead?
Both are okay. Place of adverbs in Vietnamese is not as strict as in English.
I agree that the second arrangement does sound better and there should be multiple accepted answers as long as the sentence conveys the same meaning.