Explanation:

Crop rotation is the practice of growing a series of different types of crops in the same area across a sequence of growing seasons. Its primary benefit is improving soil fertility. By rotating crops, especially including legumes, nitrogen levels in the soil can be replenished. It also helps in breaking pest and disease cycles specific to certain crops, improving soil structure, and managing weeds. While it can indirectly reduce the need for pesticides (Option D) by breaking pest cycles and can contribute to a healthier ecosystem, its most direct and significant impact is on improving and maintaining soil fertility (Option B). It does not directly eliminate parasites (Option A) or produce a greater choice of plant products (Option C) in the sense of diversifying a single crop's output.