Final answer:
The external segments of earthworms are known as annuli, and their body's segmentation is referred to as metameres, which is a trait of segmented worms in the Phylum Annelida.
Therefore, option a is correct.
Step-by-step explanation:
The external segments of earthworms are called annuli. An earthworm's body is divided into segments known as metameres, which is a characteristic feature of the Phylum Annelida.
Segmented worms, like earthworms, are known for their metameric segmentation, meaning that their body is divided into multiple similar segments both externally and internally. Each of these segments contains elements of the worm's various physiological systems, such as the nervous, circulatory, and digestive systems. In the context of this question, metameres and segments are synonymous, referring to the repeated sections that make up an earthworm's body.