Final answer:
a) Segmentation. Segmentation is the morphological trait that has shown the most variation among protostomes.
Step-by-step explanation:
The morphological trait among protostomes that has shown the most variation is segmentation. Segmentation refers to the division of the body into repeating segments. It can be observed in various groups of protostomes, such as annelids (segmented worms) and arthropods. Segmentation provides these organisms with increased flexibility, efficiency in movement, and specialization of body parts.