The correct answer is option D, " hollow nerve cord"
Reason -
A hollow nerve cord primarily called a Dorsal Hollow Nerve cord during development from ectoderm branches and rolls into a hallow structure. In Chordate the hallow nerve lies at the top of an species or it is located near to the notochord. This nerve cord in the chordates develops into the brain and spinal cord which later on forms the part of central nervous system.