Final Answer:
Among living mollusks, the ancestral condition is most nearly approached by the class Polyplacophora. Option D is answer.
Step-by-step explanation:
The class Polyplacophora, commonly known as chitons, is considered to most closely resemble the ancestral condition among living mollusks. Chitons have a distinctive dorsal shell composed of eight separate plates (valves) that overlap like articulated armor. This arrangement is considered primitive and shares characteristics with the hypothesized ancestral mollusk.
In contrast, other classes of mollusks like Gastropoda (snails and slugs), Bivalvia (clams and mussels), and Cephalopoda (squids and octopuses) exhibit more specialized features that have evolved over time.
Option D (Polyplacophora) is the answer.