Nitrogen is the most abundant element present in the earth’s atmosphere. It exists in the form of gas. This gas cannot be directly utilized by the plants hence leguminous bacteria convert nitrogen gas into ammonia by a process called nitrogen fixation. Some other bacteria convert ammonia into the compounds of nitrate and nitrite. These compounds are readily absorbed by the plant roots to synthesis proteins in their body. Nitrogen is then passed at each trophic level via food chain. When consumers die, decomposers give back the nitrogen to soil in the form of ammonia. Some bacteria may also convert nitrate into nitrogen gas by denitrification.
Bacteria that live on the roots of plants cannot break down nitrogen compounds into nitrogen gas. Hence, the option (a) is incorrect.
Some bacteria may also convert nitrate into nitrogen gas by denitrification. But this is not the function of bacteria that live on the roots of plants. Hence, the option (b) is incorrect.
Bacteria that live on the roots of plants cannot change nitrogen gas into plant proteins. Hence, the option (c) is incorrect.
Hence, the correct answer is option (d) change nitrogen gas into ammonia.