Why D? And why not the others?

Correct answer: D. About 3,700 meters

Scientists from NOAA and the Woods Hole Oceanographic Institution used satellite measurements in 2010 to calculate an average ocean depth of 3,682 meters — about 3.7 kilometers, roughly twice the depth of the Grand Canyon. The figure near 11,000 meters is the classic trap: that is the ocean's maximum depth, at the Challenger Deep in the Mariana Trench, not its average. Guesses around 1,200 meters or 800 meters feel reasonable because nearly all familiar marine life, diving records, and underwater photography come from the sunlit and twilight layers above 1,000 meters — but that thin, bright zone is the exception, not the rule. The typical patch of seafloor sits in permanent darkness kilometers down, which is a big part of why so much of the deep ocean remains unexplored.

Source: NOAA Ocean Exploration: 'How deep is the ocean?' (oceanexplorer.noaa.gov/facts/ocean-depth.html)