Q.Find the organism and also X, Y, Z.

This is the generalised diagram of a "typical chordate" body plan: the dorsal, hollow nerve cord (X) lies above the notochord (Y), a row of pharyngeal gill slits lies below them, and the mouth (Z) opens at the anterior end near the buccal region — option (b).

The defining features of Phylum Chordata are: a notochord, a dorsal hollow nerve cord, paired pharyngeal gill slits, and a post-anal tail at some stage of life. In a longitudinal diagram of a typical (generalised) chordate:

• The dorsal tubular structure closest to the back (uppermost of the two parallel tubes) is the nerve cord — it is dorsal and hollow, unlike the solid, ventral nerve cord of non-chordates. This is X.
• Just below the nerve cord runs the notochord, a flexible, rod-like structure that provides the primitive internal skeletal support and lies ventral to the nerve cord. This is Y.
• The row of small openings connected to "Pharyngeal gill slits" confirms the pharyngeal region, another chordate hallmark. …