The body of ______ is unsegmented with a distinct head, muscular foot and visceral mass 

Mollusks are characterized by a soft, unsegmented body divided into three main parts:

• Head: Contains sensory organs and the mouth.
• Muscular Foot: Used for locomotion.
• Visceral Mass: Houses internal organs such as the heart, digestive, and reproductive systems.

This body plan is typical of mollusks, distinguishing them from other invertebrates.

The other options are not mollusks and do not share this body structure:

• Asterias: A genus of starfish (echinoderms) with a radially symmetrical, segmented body.
• Ophiura: A genus of brittle stars (echinoderms) with a distinct central disc and long, slender arms.
• Balanoglossus: An acorn worm (hemichordate) with a body divided into proboscis, collar, and trunk.

Therefore, among the given options, only mollusks have an unsegmented body with a distinct head, muscular foot, and visceral mass.

Final Answer

The correct answer is d) Mollusca.