The oceans cover around 70 percent of the Earth's surface - so at least two-thirds (the other options one half and one-forth are out, since they're less than that)
The different sources mention 70, 71 and 72 percent - but they refer mostly to the oceans.
The exact answer is problematic, as it's only an estimation and it depends also on seasonal rivers which are not stable - but since even 72% of earth is covered by oceans, then i would say we can add the lakes and rivers and we will end up with 75% - answer d) (but i can imagine that some tests, depending on their sources want you to answer b)
