Final answer:
The correct option in the final answer is C. The excretory structure of mollusks that reabsorbs nutrients and excretes fluid waste is the nephridia (option C). Nephridia are tubular organs that filter waste from body fluids, akin to the role of kidneys in higher animals.
Step-by-step explanation:
Among the options given, the correct answer is C. nephridia. The nephridia are tube-shaped organs in mollusks that are responsible for the reabsorption of nutrients and the excretion of fluid waste. These excretory structures filter waste from body fluids and release the waste into the coelom. In aquatic mollusks, the nephridia help with osmoregulation and remove nitrogenous wastes, playing a key role in maintaining homeostasis.
The foot (A) is typically a muscle used for locomotion, the radula (B) is a tooth-like structure used for feeding, and the siphon (D) is involved in water intake and expulsion as part of the feeding and respiratory system, particularly in bivalves.
As seen in annelids such as earthworms, the nephridia exhibit a similar function to mollusk nephridia, filtering coelomic fluid and facilitating tubular reabsorption. Ultimately, the correct option for the mollusk's excretory structure where fluid waste is excreted and nutrients are reabsorbed is option C, nephridia.