Answer:
Time, t = 0.104 seconds
Step-by-step explanation:
Frequency of the click of the Dolphin, f = 55.3 kHz
A dolphin sends out a series of clicks that are reflected back from the bottom of the ocean 80 m below, d = 80 m
The speed of sound in seawater is, v = 1530 m/s
Once the sound is send and reflects, the total distance covered by it is 2d such that,

So, the time elapses before the dolphin hears the echoes of the clicks is 0.104 seconds.