The correct answer is option 2, Protochordata. This group includes marine animals that are bilaterally symmetrical, have a coelom (body cavity), and possess a notochord (a flexible rod-like structure providing support). However, these animals do not develop a vertebral column, which is a defining feature of the subphylum Vertebrata.

Let`s further explore the other options to understand why they are not the correct choices:

- Option 1, Chordata, is a broader group that includes all animals possessing a notochord at some point in their life cycle. While Protochordata is a member of the Chordata group, not all Chordata organisms are Protochordates.

- Option 3, Vertebrata, encompasses animals that possess a backbone or vertebral column, which differentiates them from Protochordates. Therefore, animals without a vertebral column cannot be classified as Vertebrata.

- Option 4, Mammalia, is a class within the subphylum Vertebrata. Mammals are a specialized group characterized by features such as mammary glands and hair, but not all animals within the subphylum Vertebrata are mammals.