Afishing boat using sonar to locate fish is employing
diffraction
reflection
refraction
interference

reflection

Explanation:

SONAR - Sound Navigation and Ranging

It works on the principle of Echo i.e. the reflection of sound. Sound waves are sent out which get bounced off a nearby object. The reflected sound waves is detected and distance to the object is estimated. Sonar is used to detect, navigate or communicate  objects underwater. Thus, a fishing boat uses sonar to locate the fish. It is employing reflection.

i would say the answer is food.

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