Cycas has small specialised roots known as [[1]] that are associated to nitrogen-fixing cyanobacteria.
 

Concept: Coralloid roots are specialised roots associated with nitrogen-fixing cyanobacteria found in Cycas. The cyanobacterial zone, which is inhabited by cyanobacteria, is found within the coralloid root. 
This has special properties that make a close association between cycad and cyanobacteria possible.
Hence, Coralloid roots is the correct answer.