which port scanning technique cannot be used?

Correct answer is B:

— TCP Full Connect. This mode makes a full connection to the target’s TCP ports and can save any data or banners returned from the target. This mode is the most accurate for determining TCP services, but it is also easily recognized by Intrusion Detection Systems (IDS).
TCP SYN Half Open. This mode sends out a SYN packet to the target port and listens for the appropriate response. Open ports respond with a SYN|ACK and closed ports respond with ACK| RST or RST. This mode is less likely to be noted by IDS, but since the connection is never fully completed, it cannot gather data or banner information.