Which type of nitrogen-fixing bacteria are found in the root nodules of non-leguminous plants? 

Frankia is a type of nitrogen-fixing bacteria that is found in the root nodules of non-leguminous plants, such as alder and cypress. Unlike Rhizobia, which is found in the root nodules of leguminous plants, Frankia has a broader range of plant hosts. 

Azotobacter and Cyanobacteria are also nitrogen-fixing bacteria, but they are not commonly found in root nodules. Therefore, the correct answer is - Frankia.