I'd say b. X-rays even though gamma rays are the shortest.
The answer is X-ray, I just took the test!ย
Hope this helps! :)
Hierarchical Classification
The levels of classification he used are: kingdom, phylum, class, order, family, genus, and species. You can see that genus and species are the two most specific categories, which is why they are used in binomial nomenclature to identify an organism.