The correct answer is A. Urea.
Urea is a nitrogen-rich fertilizer that is commonly used in agriculture. It is a white, crystalline solid that is soluble in water. Urea is produced by the reaction of ammonia and carbon dioxide.
Ammonium nitrate is a nitrogen-rich fertilizer that is also commonly used in agriculture. It is a white, crystalline solid that is soluble in water. Ammonium nitrate is produced by the reaction of ammonia and nitric acid.
Potassium nitrate is a potassium-rich fertilizer that is also commonly used in agriculture. It is a white, crystalline solid that is soluble in water. Potassium nitrate is produced by the reaction of potassium hydroxide and nitric acid.
Ammonium phosphate is a phosphorus-rich fertilizer that is also commonly used in agriculture. It is a white, crystalline solid that is soluble in water. Ammonium phosphate is produced by the reaction of ammonia and phosphoric acid.
Of the four fertilizers listed, urea has the highest nitrogen content. Urea contains 46% nitrogen, while ammonium nitrate contains 33% nitrogen, potassium nitrate contains 13% nitrogen, and ammonium phosphate contains 18% nitrogen.