💡 Detailed Explanation
The correct choice is 'うっかり' (carelessly / thoughtlessly). Combined with '忘れる' (to forget), 'うっかり忘れる' is a highly common collocation meaning to forget something due to carelessness. Sentence translation: 'I had carelessly forgotten that there was homework.' 'すっきり' means refreshed, 'がっかり' means disappointed, and 'ゆっくり' means slowly.