Final answer:
A true coelom develops completely within the mesoderm, facilitating organ housing and nutrient circulation.
Step-by-step explanation:
The answer to the student's question is C. This means that a true coelom develops completely within the mesoderm. In eucoelomates or coelomates, a true coelom arises entirely within the mesoderm germ layer (eucoelomate). The development of this body cavity, or coelom, is important as it houses organs and allows for efficient nutrient circulation and organ protection.