💡 Detailed Explanation
The correct choice is 'そう' (forming '降りそうです' - it looks like it will rain). The grammar pattern '[Verb Stem] + そうだ' expresses immediate conjecture about a physical state or event that is about to occur. Sentence translation: 'The sky has filled with black clouds, and it seems like it will start raining at any moment.' '今にも' (at any moment) strongly collocates with '〜そうです'. 'やす' (easy to do), 'にし' (incorrect), and 'おわ' (finish doing) do not match the immediate conjectural context.