The correct answer is option 1: Bacteria. Bacteria are the organisms that reside in the roots of pulse plants to perform nitrogen fixation. Nitrogen fixation is the process by which atmospheric nitrogen is converted into a form that can be utilized by plants. Pulse plants, such as legumes, have a symbiotic relationship with certain bacteria called rhizobia.

These bacteria form nodules on the roots of the pulse plants, where they live and carry out nitrogen fixation. Inside these nodules, the bacteria convert atmospheric nitrogen into ammonia, which is then used by the plant to synthesize proteins and other essential molecules.

Option 2, Fungi, is not the correct answer. While some fungi have symbiotic relationships with plants (such as mycorrhizal fungi), they do not typically play a significant role in nitrogen fixation.

Option 3, Protozoa, is also not the correct answer. Protozoa are single-celled organisms that are not known for their ability to fix nitrogen.

Option 4, Virus, is not the correct answer either. Viruses are parasites that infect cells and their genetic material cannot replicate outside of a host cell. They do not have the capability to perform nitrogen fixation.

In conclusion, option 1, Bacteria, is the