Step 1: Total earth surface.
The earth's total surface area is about 510 million square kilometers.
Step 2: Oceans and land distribution.
- About 71% of the surface is covered by oceans (Pacific, Atlantic, Indian, Arctic, Southern).
- The remaining 29% is land (continents and islands).
Step 3: Eliminate other options.
65%, 79%, and 81% are incorrect figures. The scientifically accurate figure is 71%.
Final Answer: \[ \boxed{\text{Oceans cover 71% of earth's surface.}} \]