大切な書類をなくし___。
Q. Which ending adds the nuance of regret ('unfortunately lost')?
Translation
I (regrettably) lost an important document.
Explanation
〜てしまう expresses completion with regret or an unintended result: なくしてしまいました. 〜ておく is deliberate preparation and 〜てみる is trying something out — both clash with losing something by accident.