Answer: a. numerous difficult steps to assemble device.
Explanation:
Radiological materials are quite cumbersome to assemble and require rare materials whose sale are intensely watched by various intelligence agencies in the world. Coupled with that is the need for very skilled scientists in order to assemble the device.
All these hurdles make it so that terrorists are not able to assemble radiological devices easily which is a good thing because if they could, it would have been used to devastating effect on many parts of the world by now.