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.

explanation:

0.3x0.21x0.08kg=mass 1 sheet

mass 500 = 500xmass 1

volume pile = 0.3x0.2x0.048

density=mass/volume= 500xmass 1/0.3x0.2x0.048

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