Phylum Chordata consists of three subphylums-

Urochordata

Cephalochordata

Vertebrata

notochord, dorsal hollow nerve cord, and pharyngeal gill slits are present in all the chordates

The correct answer is option 'D', which states that all chordates possess an axial skeletal rod called the notochord. Let's break down this answer and understand why it is correct.

Chordates:
Chordates are a diverse group of animals that includes vertebrates (animals with a backbone) as well as some invertebrate groups such as lancelets and tunicates. Chordates are characterized by the presence of certain key features during their development and/or adult stage. One of these key features is the notochord.

Notochord:
The notochord is a flexible, rod-like structure that runs along the length of the body in chordates. It is present during the embryonic stage and may persist in some species throughout adulthood. The notochord provides support to the body and serves as a precursor to the backbone or vertebral column in vertebrates.

Key Points:
- The notochord is an axial skeletal rod present in all chordates.
- It is a flexible structure that provides support to the body.
- The notochord develops during the embryonic stage and may persist in some species throughout adulthood.
- In vertebrates, the notochord is eventually replaced by the vertebral column (backbone).

Importance of the Notochord:
The notochord plays a crucial role in the early development of chordates. It provides structural support and helps to define the body axis. Additionally, it serves as a signaling center, influencing the development of surrounding tissues and organs.

Evolutionary Significance:
The presence of a notochord is considered a defining characteristic of chordates. It is believed to have evolved in the common ancestor of all chordates and is still present in the early developmental stages of all chordates today. The notochord represents an important step in the evolution of vertebrates and the development of a more complex skeletal system.

In conclusion, the correct answer is option 'D', which states that all chordates possess an axial skeletal rod called the notochord. The notochord is a key feature of chordates, providing support to the body and influencing the development of surrounding tissues.