Blunders from the 20th Century, #4

[Javier]

Level: Easy.

In the game Van Wely - Kramnik, From the Biel (Zonal) tournament of 1993, the players reached the following position:

 

 

 

With black to move, he can:

 

a) Play the sensible 1...Nb8, getting ready for the re-allocation of the Knight to c6, which is far more effective than it is at its current position on d7.

 

Try and grab the initiative by means of 1...Rg4, attacking the pawn on e4.

 

Which one would you choose? and, more importantly, why?