Suppose you wanted to do a transaction from a remote client to a server as fast as possible. would you use udp or tcp? why?

You would use UDP. With UDP, the transaction can be completed in one roundtriptime (RTT) - the client sends the transaction request into a UDP socket, and theserver sends the reply back to the client's UDP socket. With TCP, a minimum of twoRTTs are needed - one to set-up the TCP connection, and another for the client to send the request, and for the server to send back the reply