Final answer:
Most of the mass of organic material of a plant comes from d.carbon dioxide (CO2) in the atmosphere, utilized during photosynthesis. Other elements like nitrogen and soil minerals provide nutrients but do not make up the majority of the plant's mass. The main water source for land plants is soil, which provides essential hydration and nutrients.
Step-by-step explanation:
The main source of the mass of organic material in a plant comes from carbon dioxide (CO2) which is taken from the atmosphere. Plants utilize CO2 in the process of photosynthesis to produce carbohydrates, which are a significant component of plant biomass. Therefore, the correct answer to the student's question is d) carbon dioxide.
As for other options provided: Water (e) is essential for the plant's biological processes, but it is not the primary source of the organic mass; nitrogen (a) is a vital nutrient for plants but not the main contributor to their biomass; soil minerals (b) provide essential nutrients, but again, these do not make up the majority of the plant's organic mass; and atmospheric oxygen (c) is a byproduct of photosynthesis, but does not substantially contribute to the plant's organic material.
The main water source for land plants is b) soil, which acts as a reservoir of moisture that roots can absorb. Soil is also crucial because it supplies plants with essential nutrients and serves as a medium for support and growth.