The tentacles of a squid are appendages that have been modified from which primitive molluscan structure?

The correct answer is foot.

Mollusks have more diverse forms in comparison to any other animal phylum. They comprise, the slugs, snails, and other gastropods; squids and other cephalopods; clams and other bivalves; and other lesser known but identically different subgroups.  

The cephalopods like octopuses and squids possess the head, which is prominent and the foot that has been amended into tentacles. The cephalopods have an absent or diminished shell and exhibit the most advanced nervous system of the invertebrates.