Just to add a few notes in addition to Pieter's answer, and with a hint to your implied question (can AlphaGo beat Lee Sedol?):
First off, I'm not sure if you thought being 9p automatically means being stronger than 2p. This is not the case.
However, in this particular instance, other matters are of more importance. Fan Hui has not been competing in Asian (...
I believe your information comes from this unofficial rating system:
Fan Hui's page on that site:
Fan Hui only only played a few pro games - none recently - so his ranking there (or any other pro ranking system) is unreliable.
1P-9P rankings are not ratings (you only go up) and don't ...
I will concentrate on the question 3 vs. a. While a move at a will also settle black's top left group, it helps the two white stones to make shape and build up pressure towards blacks top right corner. It also takes out other possibilities of attacking the two white stones like building a black wall towards the centre. On the other hand, 3 is a perfect move, ...
Is there any official and accessible world ranking ?
No; Rankings are done by national Go organizations, with some variance existing between them.
What is the exact rank of Fan Hui ?
As reported here,
Fan [Hui] earned a 2-dan professional ranking in China before emigrating to France, so it appears that go software has reached the level of ...