💡 Detailed Explanation
The Na-adjective/noun '危険' (danger/hazardous) is read as 'きけん' (kiken). Sentence translation: 'Swimming in this river is dangerous.' 'きげん' (mood/health 機嫌), and choices starting with 'あぶ' (mixing with the kunyomi of '危ない' - abunai) are incorrect.