The correct answer is option 2: Phosphorus. Bone is used as a fertilizer because it contains high levels of phosphorus, which is an essential nutrient for plant growth. Phosphorus plays a crucial role in plant development and metabolism, particularly in energy transfer and the formation of DNA, RNA, and ATP. It also contributes to root development, flowering, and seed production. Bone meal, a type of bone fertilizer, is made from finely ground animal bones and is commonly used to enrich garden soil and promote healthy plant growth. While other nutrients such as nitrogen, sodium, and calcium are also present in bones, it is the phosphorus content that makes them particularly beneficial as a fertilizer.