Which of the following describes a 'synchronous' process?

A 'synchronous' process is characterized by the requirement for processes to wait for each other to complete their tasks before proceeding further. This behavior ensures that tasks or operations are coordinated and can rely on the results of prior tasks, maintaining a specific order of execution. In such a model, if one process needs the output or the completion status of another process, it must pause its execution until that process has finished, thus establishing a dependency between them.

The other formulations depict different interactions of processes: simultaneous execution suggests an asynchronous nature where processes run independently, while responding to signals emphasizes a possible event-driven approach rather than synchronous waiting. The notion of operating without external commands also does not connect with the synchronous concept, as it implies more of an autonomous operation rather than one influenced by other processes. Therefore, the definition aligning with processes requiring completion time of each other is indeed the essence of what makes a process synchronous.