The correct option is B. converting ammonium salt to nitrate. Detailed Explanation: Nitrifying bacteria play a crucial role in the nitrogen cycle, which is essential for maintaining soil fertility. Let's break down the process and understand why option B is correct and the others are not.

1. Understanding Nitrification:
2. Nitrification is a two-step process carried out by specific types of bacteria in the soil. It involves the conversion of ammonia (NH₃) or ammonium ions (NH₄⁺) into nitrates (NO₃⁻), which plants can readily absorb and use for growth.
3. The two main groups of nitrifying bacteria involved in this process are:
   • Ammonia-oxidizing bacteria (AOB): These bacteria convert ammonium ions into nitrites (NO₂⁻).
   • Nitrite-oxidizing bacteria (NOB): These bacteria then convert nitrites into nitrates.
4. Why Option B is Correct:
5. Option B states that nitrifying bacteria convert ammonium salt to nitrate. This is precisely what happens during the nitrification process. Ammonium salts, which can come from organic matter decomposition or fertilizers, are transformed into nitrates, making nitrogen available to plants.
6. This conversion is vital because nitrates are a primary source of nitrogen for plants, which is necessary for synthesizing proteins and nucleic acids.
7. Why the Other Options are Incorrect:
8. Option A: Converting nitrate to nitrogen:
   • This option describes a process known as denitrification, not nitrification. Denitrifying bacteria convert nitrates back into nitrogen gas (N₂), which is released into the atmosphere. This process reduces the amount of nitrogen available in the soil, which is the opposite of what nitrifying bacteria do.
9. Option C: Converting atmospheric nitrogen to plant nitrogen:
   • This process is known as nitrogen fixation, which is primarily carried out by certain bacteria (like Rhizobium) and some cyanobacteria. These organisms convert atmospheric nitrogen (N₂) into ammonia, which can then be used by plants. Nitrifying bacteria do not perform this function; they work with already fixed nitrogen (in the form of ammonium).
10. Option D: Converting nitrate to nitrous oxide:
    • Similar to option A, this describes a denitrification process where nitrates are converted into nitrous oxide (N₂O) or nitrogen gas. This is not the role of nitrifying bacteria, which focus on converting ammonium to nitrate.

Summary of Key Points:

• Nitrifying bacteria convert ammonium salts into nitrates, which are essential for plant growth.
• The nitrification process involves two steps: ammonium to nitrite and then nitrite to nitrate.
• Denitrification is the process of converting nitrates back to nitrogen gas, which is not the role of nitrifying bacteria.
• Nitrogen fixation is the conversion of atmospheric nitrogen to ammonia, a different process involving other types of bacteria.

Revision Summary:

• Nitrifying bacteria convert ammonium salts to nitrates, enhancing soil fertility.
• The nitrification process consists of two steps: NH₄⁺ → NO₂⁻ → NO₃⁻.
• Denitrification and nitrogen fixation are different processes involving other bacteria.
• Understanding these processes is crucial for grasping the nitrogen cycle and its importance in agriculture and ecology.