💡 Detailed Explanation
The correct choice is 'うっかり' (carelessly / by mistake / thoughtlessly) because it expresses doing something due to a lack of attention or carelessness. Sentence translation: 'I completely forgot an important promise by mistake.' 'がっかり' means feeling disappointed. 'しっかり' means firmly, reliably, or tightly. 'はっきり' means clearly or distinctly.