Answer:
a. Exaptations
Step-by-step explanation:
An exaptation is a structure of an organism that originally evolved as a feature to provide adaptation to a certain condition, but which, once consolidated (for example, several million years later) begins to be used and perfected in pursuit of a new purpose, which, sometimes, is not related at all to its original function.
An example is the feathers of birds. Feathers are an exaptation becasue they were originated to maintain body temperature more efficiently. However, feathers became later fundamental as a flying structure.