If you are trying to detect planes or clouds approaching you would use a , but for detecting things underwater you would use a .

Radar

Sonar

Explanation:

If one is trying to detect planes or clouds approaching, you would use a radar but for detecting things underwater, sonar is used.

Radar is a detection system that simply uses radio-waves to detect nearby objects.

A radar system is made up of transmitter which sends out the radio-waves and a receiver which processes and interprets the incoming radiations. They are extensively used in airports.

Sonar is used for echo sounding in water. It simply means Sound Navigation Ranging. It uses the properties of sound waves for navigation within water bodies. They are useful for detecting underwater materials or substances.

the answer is b i think