Final answer:
The nephridia is the tube-like structure in mollusks responsible for the reabsorption of nutrients and excretion of fluid waste.
Step-by-step explanation:
The part of the mollusk's body where nutrients are reabsorbed and fluid waste is excreted is called the nephridia. These are tube-shaped excretory organs that filter waste from body fluids. Specifically, the nephridia filter fluid from the coelom, the body cavity. As the filtrate moves through the nephridia's tubules, nutrients and other solutes are reabsorbed by the capillaries surrounding them. Finally, the filtered fluid that contains nitrogenous and other wastes is disposed of through the nephridiopore.
In contrast, siphons are involved in the intake and expulsion of water in bivalves, assisting in feeding and respiration rather than waste removal. The radula is a feeding organ present in certain mollusks, used for scraping or cutting food before ingestion, not for excretion. Lastly, the foot is a muscular structure used for motility and anchorage, but it does not have a role in nutrient reabsorption or waste excretion.