What do viroids and prions have in common?

I would say that d is probably the answer. Both can be considered pathogenic molecules, with viroids being bare nucleic acids (to be a virus, you need both a genome and a protein coat) and prions being proteins. I know for sure viroids can infect plants... not sure if they infect other organisms. Prions can infect humans, cows, deer... maybe other mammals too. They also are known to infect fungi. Pathogenic prions cause degenerative brain disease in mammals. I don't think viroids are known to infect mammals. So that would leave d as the only answer.