💡 Detailed Explanation
The correct sentence order is: 'この川は 深い(1) ので(2) 泳ぐ(0) のは(3) 危険です。' (Because this river is deep, swimming in it is dangerous). Grammatical breakdown: '[Adjective/Verb Plain Form] + ので' indicates a cause or reason (because it is deep). '泳ぐのは' (swimming [as a topic/subject]) is the nominalized phrase ('泳ぐ' + noun-maker 'の' + topic marker 'は') which is described as '危険です' (dangerous). The third position (★) is filled by '泳ぐ' (Choice '泳ぐ').