Vertebrates belong to phylum chordata because:

Explanation

Vertebrates belong to phylum Chordata primarily because they exhibit several defining features, including a vertebral column (backbone), a brain protected by a skull, and the presence of gill structures during embryonic development. Each of these characteristics is critical for the classification of organisms within this phylum. Option A, stating they have a vertebral column, is true but incomplete on its own. Option B highlights the protective feature of the skull, which is also true but does not encompass all the defining traits. Option C mentions the embryonic gills, which is a trait present in many vertebrates, but again does not capture the full spectrum of characteristics. Therefore, Option D, 'All of these', is the correct answer as it encompasses all the essential features that classify vertebrates in the phylum Chordata.