Introduction:
Vertebrates are a group of animals that possess a vertebral column or backbone. They include fish, amphibians, reptiles, birds, and mammals. These animals share certain characteristics that distinguish them from other groups of animals.

Main Body:

Characteristic of vertebrates without exception:

The correct answer to the given question is option 'D', which states that the common characteristic of all vertebrates without exception is the presence of a well-developed skull.

1. Presence of a well-developed skull:
- All vertebrates possess a well-developed skull, which is a bony structure that encloses and protects the brain.
- The skull is made up of several bones and provides a rigid framework for the protection of the brain and sensory organs such as the eyes, ears, and nasal passages.
- The skull also plays a crucial role in the attachment of muscles responsible for chewing, biting, and other movements of the head.

2. Importance of a well-developed skull:
- The presence of a well-developed skull is essential for vertebrates as it protects the delicate brain from injury.
- The skull also provides support and protection to the sensory organs, which are vital for survival and adaptation to the environment.
- Additionally, the attachment points for muscles on the skull allow for the complex movements of the head and jaw, enabling various functions such as feeding, communication, and defense.

3. Differences in skull structure:
- While all vertebrates possess a well-developed skull, there are variations in its structure among different groups.
- Fish have a relatively simple skull, consisting of several bony plates and a cartilaginous framework.
- Amphibians and reptiles have more complex skulls, with additional bones and structures to accommodate their specific adaptations.
- Birds have a highly specialized skull, designed for lightweight and efficient flight.
- Mammals have the most diverse and complex skulls, reflecting their wide range of adaptations and lifestyles.

Conclusion:
The presence of a well-developed skull is a common characteristic of all vertebrates without exception. This structure protects the brain, supports sensory organs, and allows for various head and jaw movements. While the overall structure of the skull may vary among different groups of vertebrates, its importance in providing protection and facilitating essential functions remains constant.