Which organisms transform nitrogen to a form that is useful to plants

The answer is nitrogen-fixing bacteria.

Plants are not able to directly use atmospheric nitrogen. It must be converted into different form through a nitrogen cycle. In the process of nitrogen fixation, nitrogen fixing bacteria converts atmospheric nitrogen into ammonium. In the process of assimilation, plants incorporate ammonium into proteins.

Some legumes have a symbiotic relationship with bacteria that live in their root system. These bacteria are called Rhizobia and have the ability of nitrogen-fixation.

The answer to this question is "the nitrogen-fixing bacteria"

Nitrogen-fixing bacteria are a type of organisms that transform nitrogen to a form that is useful to plants. These microorganisms are capable of transforming atmospheric nitrogen into fixed nitrogen.