Rhizobium:

are nitrogen-fixing bacteria
live in the nodules of legumes
convert nitrogen in soil into nitrogen compounds
all of the above

All of the above

Explanation:

Rhizobium is an important diazotrophic bacteria in nitrogen cycle. They are involved in a symbiotic relationship with leguminous plants; they assist in nitrogen fixation in the roots of these plant by taking up nitrogen from the atmosphere and passing it into the plants (in the form of ammonia or amino acids) which allows the plant to grow in soil with low nitrogen. The plant in exchange provide organic acids for the bacteria which it uses for carbon and energy source.

From the above, we can deduce that

(a) rhizobium are nitrogen fixing bacteria

(b) they live in the nodules of legumes (hence the symbiotic relationship)

(c) convert nitrogen in soil into nitrogen compounds like ammonia or amino acids

hence, the correct option is all of the above

the correct answer would be d. hope this !