The series of repeated divisions in the body are called segments. It is first observed in Annelids(e.g. Earthworm).
The body of earthworm is divided into segments both internally and externally which we call as true segments.
In Aschelminthes(e.g. Tape worm) we could find false segments(pseudosegmentation) which is not much prominent. Hence, the Annelids hold the position of having true segments.
Hence, option C is right.

Segmentation in the body is first observed in Annelida.

Annelida:
Annelida is a phylum of segmented worms that includes earthworms, leeches, and marine worms. These organisms exhibit true metamerism, which is the presence of a series of similar body segments along the length of the body. Each segment contains a set of organs and structures, and the body is divided into several repeating units called metameres.

Advantages of Segmentation in Annelids:
1. Improved locomotion: The segments in annelids are equipped with setae, which are bristle-like structures that aid in movement. The coordinated movement of these segments allows for efficient burrowing, crawling, and swimming.
2. Redundancy of organs: Each segment in an annelid possesses a complete set of organs. This redundancy ensures that even if one segment is damaged or lost, the organism can still survive and function.
3. Specialization of segments: Different segments of the annelid body may be specialized for specific functions. For example, the first few segments of an earthworm are modified for sensing the environment, while the middle segments are involved in digestion and absorption, and the posterior segments aid in reproduction.
4. Regeneration: Segmentation enables annelids to regenerate lost segments or even the entire body. This remarkable regenerative capacity allows them to recover from injuries or predation.

Comparison with other Phyla:
- Aschelminthes (also known as Nematoda) are unsegmented roundworms. They lack the true metameric segmentation seen in annelids.
- Arthropoda, which includes insects, spiders, and crustaceans, exhibit tagmatization, where the body is divided into specialized regions called tagmata (e.g., head, thorax, abdomen), but not true segmentation.
- Platyhelminthes (flatworms) are also unsegmented organisms.

In conclusion, segmentation is first observed in the body of annelids. This unique characteristic provides numerous advantages, such as improved locomotion, redundancy of organs, specialization of segments, and the ability to regenerate.