What does a NULL scan send?

The correct answer is A, No flags set. In CEH scanning concepts, a NULL scan is a TCP port-scanning technique in which the scanner sends a TCP segment with none of the TCP flags enabled. This means the packet does not contain SYN, ACK, FIN, RST, PSH, or URG flags. The uploaded CEH material states that when no TCP flag is set, the scan is known as Null Scanning, and further describes a NULL scan as sending a packet “without any flag set.” This differs from a SYN scan, which sends SYN packets to begin a half-open connection, and from an ACK scan, which sends packets with the ACK flag set to help identify filtering behavior. RST is not what a NULL scan sends; rather, a closed port may respond with RST. For exam purposes, remember: NULL scan equals empty TCP flag field, so the best answer is No flags set.