Prokaryotic autotrophic nitrogen-fixing symbiont is found in:

Step-by-step solution

Cycas forms facultative symbiotic association with autotrophic nitrogen fixing cyanobacteria. Cycas provide fixed carbon and a stable environment to the cyanobacteria in exchange for fixed nitrogen. These cyanobacteria are endosymbionts and live within the roots of Cycas . In addition to normal roots, Cycas develops specialised symbiotic organs at a young age called precorallioa roots which transform into coralloid roots upon successful colonisation by cyanobacteria.