The establishment of a TCP connection involves a negotiation called three-way handshake. What type of message does the client send to the server in order to begin this negotiation?

ACK is used in the third step of the handshake to acknowledge the server's SYN-ACK response, not to initiate the connection.

The client starts a TCP connection by sending a segment with the SYN flag set, which signals the intent to synchronize sequence numbers and establish a session. The server responds with SYN-ACK (step 2), and the client completes the handshake with a final ACK (step 3), after which data transfer can begin.

RST (reset) is used to abruptly terminate an existing or unwanted connection, not to begin a negotiation.

SYN-ACK is the server's second-step response to the client's initial SYN, not the message the client sends to begin the handshake.