In the nutrient cycle bacteria that live on the roots of plants a. break down nitrogen compounds into nitrogen gas b. denitrify nitrogen compounds c. change nitrogen gas into plant proteins d. change nitrogen gas into ammonia

The bacteria that lives on the roots of plants is called nitrogen fixing bacteria.
This type of bacteria turns nitrogen gas into nitrogen compounds that plants can use - this can also occur by the action of lightening as the energy from lightning makes nitrogen react with oxygen in the air to give nitrates.

So the answer would be C, I think.