Final answer:
Chlamydia trachomatis is classified at the species level in the taxonomic hierarchy, and the fact that Chlamydia can only survive as intracellular parasites supports the regressive hypothesis of viral origin.
Step-by-step explanation:
In the taxonomic hierarchy, Chlamydia trachomatis belongs to the level of species among Chlamydia. The current taxonomic system consists of eight levels, with species being the most specific level. Therefore, the correct answer to which taxonomic level Chlamydia trachomatis belongs is d. Species.
Regarding which viral origin hypothesis is supported by the fact that the genus Chlamydia contains species that can only survive as intracellular parasites, the correct hypothesis would be b. Regressive. This is because the regressive hypothesis suggests that viruses may have originated from more complex organisms that lost functions over time as they adapted to a parasitic lifestyle.