💡 Detailed Explanation
The correct choice is '突然' (totsuzen - suddenly / unexpectedly). The adverb '急に' (kyū ni) refers to a sudden occurrence without advance warning, which matches '突然'. Sentence translation: 'Suddenly, a heavy rain started falling.' 'しばらく' means for a while, 'やっと' means at last, and 'そろそろ' means soon.