Introduction:
Biofertilisers are organisms that enhance the nutrient content of the soil. They play a crucial role in sustainable agriculture by promoting plant growth and improving soil fertility. Among the options provided, all of them can be used as biofertilisers.

Nitrogen-fixing cyanobacteria:
Cyanobacteria are photoautotrophic organisms capable of fixing atmospheric nitrogen into a usable form for plants. They possess specialized structures called heterocysts where nitrogen fixation occurs. Cyanobacteria such as Anabaena, Nostoc, and Spirulina are commonly used as biofertilisers. They colonize the rhizosphere of plants and convert atmospheric nitrogen into ammonia, which can be readily absorbed by plants.

Nitrogen-fixing bacteria:
Certain bacteria, such as Rhizobium and Azotobacter, have the ability to fix atmospheric nitrogen into a form that can be utilized by plants. Rhizobium forms a symbiotic relationship with leguminous plants, forming nodules on their roots. These nodules provide a favorable environment for the bacteria to convert nitrogen gas into ammonia. Azotobacter, on the other hand, colonizes the rhizosphere and fixes nitrogen in the soil.

Mycorrhizae:
Mycorrhizae are mutualistic associations between fungi and plant roots. They form a symbiotic relationship where the fungus provides essential nutrients, particularly phosphorus and micronutrients, to the plant while receiving carbohydrates from the plant. Mycorrhizal fungi enhance nutrient uptake, improve soil structure, and protect plants against pathogens. They are widely used as biofertilisers to improve plant growth and nutrient absorption.

Conclusion:
In conclusion, all of the options provided, namely nitrogen-fixing cyanobacteria, nitrogen-fixing bacteria, and mycorrhizae, can be used as biofertilisers. These organisms contribute to the enrichment of soil nutrients, particularly nitrogen and phosphorus, which are crucial for plant growth. By utilizing biofertilisers, farmers can reduce their dependence on synthetic fertilisers and promote sustainable agricultural practices.