Solution:Concept: In order to store nitrogen from the atmosphere in the roots of the leguminous plants, several bacteria have genes for nitrogen fixing. And the plants utilise this nitrogen to fuel their expansion. The plants then provide them with the nutrients.
The 'Nif' gene in their nucleic acid gives them the ability to fix nitrogen. The "Nif" gene is located on the rhizobium's plasmid. The plant in turn offers the rhizobium bacteria a place to flourish. As a result, there is a symbiotic relationship between these two creatures. They are referred to as symbionts. As a result, legumes' roots contain specialised bacteria that fix nitrogen.
Hence, option 1 is the correct answer.