Pulse repetition rate determines round-trip time Steps:

• Calculate pulse rate: 5 pulses in 0.8 s gives 5 / 0.8 = 6.25 Hz.
• Find time between pulses: T = 1 / 6.25 Hz = 0.16 s.
• Determine round-trip distance: 2 × 120 m = 240 m.
• Compute speed: v = 240 m / 0.16 s = 1500 m/s.

Why D is correct:

• 1500 m/s satisfies v = (2 × depth) / T, where T is the pulse interval from the repetition rate.

Why the others are wrong:

• A: 150 m/s yields T = 240 / 150 = 1.6 s, rate 0.625 Hz (too slow).
• B: 300 m/s yields T = 0.8 s, rate 1.25 Hz (too slow).
• C: 750 m/s yields T = 0.32 s, rate 3.125 Hz (still too slow).

Final answer: D