A packet containing a long string of NOP's followed by a command is usually indicative...

A series of the same control, hexidecimal, characters imbedded in the string is usually an indicator of a buffer overflow attack. A NOP is a instruction which does nothing (No Operation - the hexadecimal equivalent is 0x90)

The following answers are incorrect:

A syn scan. This is incorrect because a SYN scan is when a SYN packet is sent to a specific port and the results are then analyzed.

A half-port scan. This is incorrect because the port scanner generates a SYN packet. If the target port is open, it will respond with a SYN-ACK packet. The scanner host responds with a RST packet, closing the connection before the handshake is completed. Also known as a Half Open Port scan.

A packet destined for the network's broadcast address. This is incorrect because this type of packet would not contain a long string of NOP characters.