Concept: Urea is a nitrogen-rich fertilizer used to supply nitrogen to crops.Explanation: Urea contains a high percentage of nitrogen in its structure, CO(NH2)2. When applied to the soil, it undergoes decomposition and releases nitrogen in forms that plants can absorb easily. Nitrogen is essential for plant growth because it supports the formation of proteins, chlorophyll, and other vital compounds. Thus, urea is mainly regarded as a good source of nitrogen in the soil. Phosphorus, calcium, and sodium are not provided by urea in significant amounts.Answer: D. nitrogen in the soil.