Atuna fish is streamlined to reduce friction which describes the type of of friction the fish must overcome

A tuna fish is streamlined to reduce friction which describes the type of of friction the fish must overcome drag force. Drag force affects the fall of a tuna fish because the area of a body has an increase in surface area and hinders the flow of the molecules around it thus increasing the drag force. 

diagram is missing, but i will try to explain your doubt.

answer:

amplitude is the maximum displacement or distance moved on a point of a vibrating body or wave measured from its equilibrium position.it is equal to half the length of vibration path.

show the wavelengths of a wave. show the crests of a wave.