Explanation:

- Option 1: Ammonifying bacteria

- These bacteria break down organic nitrogen into ammonia, not nitrates into nitrogen.

- Option 2: Nitrifying bacteria

- Convert ammonia to nitrites and then to nitrates, but do not transform nitrates into nitrogen.

- Option 3: Denitrifying bacteria

- These are responsible for converting nitrates into nitrogen gas, completing the nitrogen cycle.

- Option 4: Both (a) and (b)

- Neither ammonifying nor nitrifying bacteria convert nitrates into nitrogen gas.

Correct Answer: Option 3, denitrifying bacteria