Mapping of the ocean floor

Correct answer: A. Bathymetry and sonar-based depth mapping

Explanation: The governing concept is bathymetric mapping: measuring water depth to reconstruct the hidden shape of the seabed. Echo-sounding sonar sends an acoustic pulse toward the ocean floor and records the return time. If sound speed is known or corrected for temperature, salinity and pressure, depth can be estimated from depth = sound speed × travel time ÷ 2, because the pulse travels down and back. Repeated measurements from ships, multibeam systems or autonomous platforms produce detailed maps of ridges, trenches, plains and seamounts. Option A is correct. Coastal visual observation cannot see the deep floor, while crop surveys and cloud counts measure unrelated environments.