Final answer:
The main forms of nitrogen that plants take up from the soil are nitrate, nitrite, and ammonium. All three forms are utilized by plants for growth and development, with nitrate generally being the preferred form. The correct option is d.
Step-by-step explanation:
The main forms of nitrogen taken up by plant roots and useful to plants are nitrate, nitrite, and ammonium. Nitrogen is a critical macronutrient for plants, and it is absorbed in various forms. Nitrogen fixation is a process where gaseous nitrogen is converted to a usable form, ammonia, which can occur via biological fixation by bacteria or through abiotic processes like lightning.
Ammonification is the release of ammonium ions from decomposing organic compounds. Then, nitrification follows, with bacteria converting ammonium to nitrite and then to nitrate, which are more readily absorbed by plants. While plants can take up nitrogen in different forms, most prefer nitrate. It's important to note that excessive ammonia can be toxic, and plants metabolize it quickly. Therefore, the correct answer to the question is d) All of the above.