The correct option is ( b) bacteria because it is a kind of a big teriore called rhizobium bacteria which is present in the root nodules of leguminous plants and this is help in the role of nitrogen fixation

Nitrogen is an essential element for the growth and development of living organisms. However, it exists in the atmosphere in the form of nitrogen gas (N2), which is not directly usable by most organisms. The conversion of nitrogen gas into usable forms is known as nitrogen fixation and is primarily carried out by certain microorganisms.

Bacteria are the microorganisms that play a crucial role in maintaining the balance of nitrogen gas in the atmosphere. They are capable of converting atmospheric nitrogen gas into nitrogen compounds that can be used by plants and other organisms. This process is known as biological nitrogen fixation.

Here is an explanation of why bacteria are the correct answer:

1. Biological Nitrogen Fixation: Bacteria have the unique ability to convert atmospheric nitrogen gas into a usable form called ammonia (NH3). This process is carried out by nitrogen-fixing bacteria such as Rhizobium, Azotobacter, and Cyanobacteria. These bacteria contain an enzyme called nitrogenase, which converts nitrogen gas into ammonia through a series of biochemical reactions. This ammonia can be further converted into other nitrogen compounds such as nitrites and nitrates, which can be absorbed by plants.

2. Symbiotic Relationships: Some nitrogen-fixing bacteria form symbiotic relationships with plants. For example, Rhizobium bacteria form nodules on the roots of leguminous plants such as peas and beans. These bacteria provide the plants with a source of fixed nitrogen, while the plants provide the bacteria with carbohydrates and a suitable environment for growth. This symbiotic relationship benefits both the bacteria and the plants.

3. Free-Living Bacteria: Apart from symbiotic relationships, there are also free-living nitrogen-fixing bacteria that can fix nitrogen in the soil. Azotobacter and Cyanobacteria are examples of free-living nitrogen-fixing bacteria. They convert atmospheric nitrogen into ammonia in the soil, making it available for plants and other organisms.

In conclusion, bacteria play a vital role in maintaining the balance of nitrogen gas in the atmosphere through biological nitrogen fixation. They convert atmospheric nitrogen gas into usable forms such as ammonia, which can be utilized by plants and other organisms. This process is essential for the overall nitrogen cycle and the availability of nitrogen for living organisms.