The difference in the average percentage increase in revenues, from 4th to 6th year, of Yahoo and Facebook is:

The revenue of Yahoo in 4th year = 250 millions.

The revenue of Yahoo in 6th year = 1200 millions.

Therefore, average percentage increase in Yahoo's revenue in a year = $$[(\dfrac{1200}{250})^{0.5}-1]*100$$ = 119%

The revenue of Facebook in year 4 = 300 millions

The revenue of Facebook in year 6 = 2000 millions

Therefore, average percentage increase in Facebook's revenue in a year = $$[(\dfrac{2000}{300})^{0.5}-1]*100$$ = 159%

Therefore, the required difference = 159 - 119 = 40%. Hence, option B is the correct answer.