Which adaptation is a key characteristic of annelids that greatly increases their flexibility and mobility?

The correct answer is segmentation. The annelids also known as segmented worms, are bilaterally symmetrical invertebrate organisms. Their main characteristic is a segmented body. Annelid's body consists of segments that are identical (except the first and the last one), having the same sets of internal organs and external chaetae. Septa separate the segments and enable the annelids to change the shapes of individual segments, which facilitates movement by peristalsis (symmetrical contraction movement).