Final answer:
A read-only key, in programming, is an Object that permits viewing but not modification of data.
Step-by-step explanation:
A read-only key is typically an access credential that allows users to view data without being able to modify it. In the context of programming and data structures, however, the term could be misleading as a key in this sense might refer to a unique identifier used in objects or associative arrays to access values. Given the options provided, the correct answer is that a read-only key is an Object.