Final answer:
The Chlamydiales phylum is characterized by a parasitic life cycle. They are obligate intracellular parasites, often causing diseases such as trachoma and sexually transmitted infections.
Step-by-step explanation:
The characteristic feature of the Chlamydiales phylum is B) a parasitic life cycle. While bacterial shape varies and is not phylum-dependent, the distinct life cycle of Chlamydiales sets them apart, as they are obligate intracellular parasites, meaning they can only grow within another living organism's cells. They are known for causing various diseases in humans and other animals. An example of a species in this phylum is Chlamydia trachomatis, which causes the disease trachoma and sexually transmitted infections.