The correct answer is D. No improvement.
The sentence “The partners swore from fighting again” is grammatically correct and conveys the intended meaning. The word “swore” is used in the sense of “promised solemnly,” and the phrase “from fighting again” indicates that the partners are promising not to fight in the future.
Option A, “not fighting again,” is also grammatically correct, but it is not as concise as the original sentence. Option B, “by not to fight again,” is grammatically incorrect. The word “to” is not necessary after the word “not.” Option C, “that they wouldn’t fight again,” is also grammatically correct, but it is wordier than the original sentence.
Therefore, the correct answer is D. No improvement.