Protocols in which the sender sends one frame and then waits for an…

correct answer is Option C.

This protocol is defined by its strict flow control mechanism where the sender transmits a single data frame and then halts operation to wait for an acknowledgment (ACK) from the receiver before sending the next frame. This "stop-and-wait" behavior ensures that the receiver is not overwhelmed by incoming data, distinguishing it from "unrestricted" protocols (Option B) which assume the receiver can handle data at any speed without feedback. It is termed "simplex" in this context because the model typically assumes data flows in one direction while acknowledgments flow in the other.