Question

how were the japanese able to surprise the us at pearl harbor?
japanese land forces had invaded hawaii unnoticed.
japanese submarines invaded pearl harbor unnoticed.
japanese airplanes had flown to the us mainland unnoticed.
japanese aircraft carriers had approached hawaii unnoticed.

Explanation:

The Japanese were able to surprise the US at Pearl Harbor mainly because their aircraft carriers approached Hawaii unnoticed. Japanese land forces did not invade Hawaii before the attack. Japanese submarines were part of the attack but the key element for surprise was the aircraft carriers. Japanese airplanes did not fly to the US mainland; they attacked Pearl Harbor from the aircraft carriers near Hawaii.

Answer:

Japanese aircraft carriers had approached Hawaii unnoticed.