Which of the following is a symbiotic microorganism?

Symbiotic microorganisms live in a mutually beneficial relationship with other organisms.

Rhizobium is a classic example of a symbiotic nitrogen-fixing bacterium. It lives in the root nodules of leguminous plants, where it converts atmospheric nitrogen into ammonia, which the plant can use, while the plant provides the bacteria with organic compounds and a protected environment.

Bacillus spp., Nitrosomonas, and Pseudomonas are generally free-living bacteria in the soil, with Nitrosomonas being involved in nitrification and Pseudomonas in denitrification.

Answer: Rhizobium