Which of the following is a pair of biofertilizers?

Azolla and BGA.

The correct pair of biofertilizers is "Azolla and BGA" (Blue-Green Algae).

Azolla is a floating aquatic fern that forms a symbiotic relationship with a group of nitrogen-fixing bacteria known as Blue-Green Algae (BGA) or Cyanobacteria. These cyanobacteria belong to the genus Anabaena and are capable of fixing atmospheric nitrogen into a usable form that can benefit plants.

When Azolla and BGA coexist in water bodies, the BGA residing within specialized cavities on Azolla's leaves fix nitrogen from the air and convert it into ammonia. This ammonia is then utilized by Azolla for growth and development. When Azolla is used as a biofertilizer, it can be incorporated into paddy fields or other agricultural systems, providing a natural source of fixed nitrogen to the soil. This can help enhance soil fertility, improve plant growth, and reduce the need for synthetic nitrogen fertilizers.

The other options mentioned do not represent pairs of biofertilizers:

• Salmonella and E. Coli are not biofertilizers. They are pathogenic bacteria that can cause illness in humans and animals.
• Nostoc is a genus of cyanobacteria (also known as Blue-Green Algae) that can fix nitrogen, but it does not typically form a symbiotic relationship with legumes.
• Rhizobium is a type of bacteria that forms a symbiotic relationship with leguminous plants (e.g., beans, peas, clover) and is capable of nitrogen fixation. However, it is not specifically associated with grasses.

Therefore, the correct pair of biofertilizers is "Azolla and BGA" (Blue-Green Algae).