Concept:Crop rotation is a farming practice where different crops are grown in succession on the same land to maintain soil health.Explanation:Different crops absorb and return different nutrients to the soil.Growing legumes in rotation adds nitrogen, which naturally restores soil fertility.This practice also helps reduce pests and diseases, but the main purpose among the given options is improving soil fertility.Therefore, crop rotation is practiced primarily to enhance the nutrient content and productivity of the soil.Answer:D. improve soil fertility