The correct answer is: for.
Exercise is necessary for good health. It helps to keep the body strong and healthy, and it can also help to prevent diseases such as heart disease, stroke, and diabetes. Exercise can also help to improve mental health by reducing stress and anxiety, and it can also help to improve sleep quality.
The other options are incorrect because they do not make sense in the context of the sentence. Option A, “by,” would imply that exercise is something that is done to someone, rather than something that is done by someone. Option B, “for,” is the correct answer because it indicates that exercise is something that is necessary for good health. Option C, “with,” would imply that exercise is something that is done with someone, rather than something that is done by someone. Option D, “in,” would imply that exercise is something that is done in a particular place, rather than something that is done by someone.