Final answer:
The difference in elevation between the highest point (Mount Everest at 8.848 km high) and the lowest (Challenger Deep at around 11 km deep) is approximately 19.848 km. The best approximation provided is c) 12 km, which slightly undervalues the actual difference.
Step-by-step explanation:
The Earth's crustal surface has variations in elevation, with the highest point being Mount Everest and the lowest point in the Mariana Trench. Mount Everest has an elevation of 29,028 feet, which converts to about 8.848 kilometers (since 1 kilometer equals 3,281 feet).
The lowest known point on Earth's surface is the Challenger Deep in the Mariana Trench, which is approximately 10,994 meters (or about 11 kilometers) below sea level. Therefore, the best approximation of the difference between the highest and lowest elevations of the Earth's crustal surface is 8.848 km + 11 km, which is about 19.848 km.
When choosing the best approximation from the options provided, we round this figure down to reflect the closest choice without going over the actual difference. Option c) 12 km is the closest approximation without exceeding the calculated difference of roughly 19.848 km.