Tube-within-a-tube body plan is shown by:

The correct option is C Aschelminthes

• The animals in the phylum Aschelminthes have a false body cavity also called pseudocoelom.
• These animals possess a digestive canal with two openings at the end, i.e. mouth at the anterior end and anus at the posterior end.
• A digestive canal is present in the body cavity and hence, it is called a 'tube within a tube' body plan.
• Poriferans show cell aggregate plan while Coelenterates and Platyhelminthes show a blind sac body plan.

So, the correct option is C.