Dolphins communicate using compression waves (longitudinal waves). some of the sounds dolphins make are outside the range of human hearing.
upon spotting a hungry shark, dolphin a sends a message to dolphin b. to send this message as effectively as possible, dolphin a would

Explanation:

Sound is a longitudinal wave. Sound wave consists of compression and rarefaction. It needs a medium to travel. It can travel in solid, liquid and gas. It travels faster in solid in comparison to liquid.

Dolphins communicate with each other using higher frequencies which are outside the range of human hearing.

In the given problem Dolphin A sends a message to dolphin B. Sound will travel through the water. Dolphin A sends a message to dolphin B underwater. A denser medium produces a louder sound.