In the provided sequence diagram, each call must complete before the next one can proceed, indicating that the calls are synchronous. Synchronous calls block further execution until the response is received, maintaining a sequential order of operations.

Synchronous Nature: The diagram shows each function call being made and then waiting for a callback before proceeding to the next step.

Blocking Behavior: Each interaction completes fully before the next one starts, which is characteristic of synchronous operations.

Option C correctly identifies that all the calls shown in the sequence diagram are synchronous.