The correct answer is A. Nitrogen fixing bacteria.
Nitrogen fixing bacteria are a type of bacteria that can convert atmospheric nitrogen into ammonia, which plants can use to make proteins. This process is essential for life on Earth, as it allows plants to obtain the nitrogen they need to grow.
Denitrifying bacteria are a type of bacteria that convert nitrates back into nitrogen gas. This process occurs in anaerobic environments, such as in the soil or in the ocean. Denitrifying bacteria play an important role in the nitrogen cycle, as they help to regulate the amount of nitrogen in the environment.
Nitifying bacteria are a type of bacteria that convert ammonia into nitrites and then into nitrates. This process is essential for plants, as they can absorb nitrates from the soil. Nitrifying bacteria play an important role in the nitrogen cycle, as they help to make nitrogen available to plants.
Nitrate fertilizers are a type of fertilizer that contains nitrates. Nitrates are a form of nitrogen that plants can use to make proteins. Nitrate fertilizers are often used to improve crop yields. However, excessive use of nitrate fertilizers can pollute the environment.
In conclusion, the correct answer is A. Nitrogen fixing bacteria.