Correct answer is 1- stop and wait
You must note that answer will change if it is given stop and wait( Arq) bcz stop and wait ARQ has both error and flow control. But simple stop and wait has only flow control because of chances in lost of data and ack..
However the simplest protocol which is last option is neither flow nor error control.. It is unidirectional protocol in which data move from sender to recvr..
