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

make a softer sound because the shark will not hear the softer signal.

send the message above the water when it jumps because air is more dense than water, and the sound would be louder above the water.

send the message underwater because a more dense medium would make the sound travel faster.

make a higher pitched signal because a higher pitch is a louder sound.