A program uses a List ADT backed by an array. The implementation is swapped for a linked one and not a single line of calling code changes - yet the program becomes noticeably slower. How can both facts be true at once?

D. Because the interface guarantees operations, not costs.

An ADT is a contract about behaviour, what operations exist and what they return, and cost is not in it, which is exactly why the swap compiles and why it can wreck performance. Indexing is the clearest case: list[500] is one multiplication and a read in an array, and five hundred pointer follows in a linked list. Neither structure dominates, which is why the choice is a design decision.