The correct answer is option 'C': sensory tentacles located on the posterior head region.

Explanation:

Mollusca is a phylum of invertebrate animals that includes organisms such as snails, clams, squids, and octopuses. They have various distinct characteristics that differentiate them from other phyla. Let's discuss each option and why option 'C' is incorrect.

a) Radula - rasping organ:
- The radula is a specialized feeding organ found in most mollusks except bivalves (clams, mussels, etc.).
- It is a ribbon-like structure with rows of tiny, chitinous teeth that are used for scraping food particles from surfaces.
- The radula is highly adapted to the feeding habits of different mollusks, and its function is to aid in feeding.

b) Feather-like gills - excretory function:
- Mollusks possess gills for respiration.
- Gills are specialized structures responsible for the exchange of gases (oxygen and carbon dioxide) with the surrounding environment.
- In many mollusks, the gills are feather-like in appearance, allowing a large surface area for gas exchange.
- Gills are primarily involved in respiration and not excretion.

c) Sensory tentacles located on the posterior head region:
- This statement is incorrect. Mollusks do not possess sensory tentacles on their posterior head region.
- Sensory tentacles are usually located on the anterior (front) head region of some mollusks, such as snails and slugs.
- These tentacles are equipped with sensory receptors, including chemoreceptors and touch receptors, which aid in detecting food, predators, and the environment.

d) Body of mollusk - unsegmented:
- The body of mollusks is not segmented.
- Unlike segmented worms (annelids), mollusks do not have a segmented body plan.
- Mollusks have a soft body that may be protected by a hard shell in some species.
- The body is typically divided into three main regions: the head, visceral mass (containing organs), and a muscular foot used for locomotion.

In conclusion, option 'C' is incorrect because mollusks do not have sensory tentacles located on their posterior head region. Instead, sensory tentacles are typically found on the anterior head region of certain mollusks.