Here the time taken by the sonar pulse (or ultrasound) to go from the submarine to the underwater cliff and return to the submarine is 1.02 second. So, the time taken by the sonar pulse (or ultrasound) to go from the submarine to cliff will be half of this time, which is 2 second. Now, knowing the speed of sound in water, we can calculate the distance travelled by sound in 0.51 second. This will give us the distance of the cliff from the submarine.
We know that : Speed = Distance/Time
1531 =Distance / 0.51
And, Distance = 1531 × 0.51
= 780.8 m