The correct answer is B. carbon dioxide.
Plants require sunlight, carbon dioxide, and water to carry out photosynthesis, which is the process of converting light energy into chemical energy (in the form of glucose) that the plant can use. The carbon atoms in glucose molecules come from carbon dioxide, which the plant takes in through small pores on its leaves called stomata. The plant also takes in water through its roots and absorbs nutrients and minerals from the soil. However, while soil minerals are necessary for plant growth, they make up only a small percentage of the plant's organic material. Nitrogen is also an essential nutrient, but it is used primarily to synthesize proteins and other organic molecules, not to form the bulk of the plant's mass.
Atmospheric oxygen is not a significant source of organic material for plants because plants release oxygen as a byproduct of photosynthesis, rather than taking it in. Water, while crucial for plant growth, does not contribute significantly to the mass of organic material in the plant; it is primarily used to transport nutrients and support the plant's structure. Therefore, most of the mass of organic material in a plant comes from carbon dioxide, making option B the correct answer.