Synchronous API calls are blocking operations which means the code will not execute any further until a response is received from the server. This can add perceived latency to an application because it has to wait for the data to be received before continuing (Option C). Furthermore, synchronous calls block until a response is returned from the server (Option D), which means that during this time, no other operations can be performed.
This is in contrast to asynchronous calls, which allow other processes to run while waiting for the API call to return. References := ( Automating Cisco Enterprise Solutions Official Cert Guide )