The correct option is (a).
Explanation
Rhizobium is a genus of soil bacteria capable of fixing atmospheric nitrogen. It establishes a specific biological association with leguminous plants, playing a vital role in soil fertility and the nitrogen cycle.
Statement-wise Analysis:
- Statement 1 is Correct: Rhizobium bacteria reside in the roots of leguminous plants (such as peas, beans, and lentils). They induce the formation of specialized structures called root nodules, where the bacteria proliferate and function.
- Statement 2 is Incorrect: The relationship between Rhizobium and the leguminous plant is symbiotic (mutualistic), not parasitic. In this association, both partners benefit: the bacteria provide fixed nitrogen to the plant, and the plant provides nutrients (sugars) and shelter to the bacteria. A parasitic relationship involves harm to the host, which is not the case here.
- Statement 3 is Incorrect: The primary function of Rhizobium is to convert atmospheric nitrogen ($N_2$) into ammonia ($NH_3$) or related nitrogenous compounds that the plant can absorb and utilize. It does not convert atmospheric oxygen into liquid nitrogen.
Key Takeaway:
Rhizobium facilitates biological nitrogen fixation through a mutualistic relationship with leguminous plants, converting inert atmospheric nitrogen into ammonia within root nodules.