The most significant achievement in surveillance of the enemy land forces was provided by
O Airplanes
O Submarines
O Phone Taps
O Double Agents

O Airplanes

Explanation:

Airplanes provided the most significant surveillance of the enemy land  forces because through aeroplane we can see all the activities carried out by enemies against us. There are certain devices and instruments which can be installed on the aeroplane in order to collect data about the surveillance of the enemy land  forces when the airplanes passes over the area of the enemies.

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