Which technique was useful in post-drift studies for knowing ocean floor depth and structure?

Correct answer: Sonar survey

The direct answer is A: sonar surveys were useful for learning about the depth and structure of the ocean floor. Sonar means sending sound waves through water and measuring the returning echo. If the echo returns quickly, the surface is relatively near; if it takes longer, the surface is deeper. Repeated measurements from ships helped scientists prepare maps showing underwater ridges, trenches, plains and other features. These maps became important evidence in post-drift studies because they revealed the shape of mid-ocean ridges and helped support ideas about seafloor spreading. Option A is correct because sonar is an actual marine surveying technique. Option B is wrong because a plough is an agricultural tool used on land. Option C is wrong because a loom is used to make textiles. Option D is wrong because a sundial measures time from the position of the Sun and cannot map deep ocean surfaces. Sonar does not directly show every internal rock detail, but it is highly useful for measuring depth and mapping seafloor relief. Memory cue: sound echo in water = sonar; sonar helps map the ocean floor.