The correct answer is option 2: Nitrogen. Plants require various essential elements for their growth and development. While all of the listed options are important for plant nutrition, nitrogen is required in the largest quantity. Nitrogen is a vital component of proteins, enzymes, and chlorophyll, which are essential for various physiological processes in plants, including photosynthesis, growth, and reproduction. It plays a crucial role in the development of leaves, stems, and roots. Nitrogen is a major nutrient that plants obtain from the soil in the form of nitrates or ammonium ions. However, it is important to note that other elements, such as calcium, phosphorus, and sulfur, are also required in significant amounts by plants, but not to the same extent as nitrogen.