Tube -within -tube body plan is shown by

The correct option is A Annelida
Tube - within - tube body plan is shown by phylum Annelida.

These are the characteristics of Annelida:

• Free-living, terrestrial or aquatic form (freshwater or marine).
  
• The body is long, cylindrical and metamerically segmented.
  
• Body wall consists of the cuticle, epidermis and musculature.
  
• The body cavity is a true coelom, lined by epithelium.
  
• The digestive system is complete.
  
• Respiration is either through the skin or through gills.
  
• Excretion with the help of characteristic nephridia.
  
• The presence of a circulatory system (closed type). Hemoglobin dissolved directly in the plasma.
  
• The nervous system consists of a nerve ring and a double ventral nerve cord with segmental ganglia.
  
• Locomotion with the help of setae or parapodia.
  
• Reproduction by sexual methods. Either unisexual or bisexual development may be direct or indirect.
  
• The body has a true coelom.