Definitely the most natural reply, since the b4-square can be used by Black's pieces.
1. e2e4  1... e7e6  2. d2d4  2... d7d5  3. e4e5  3... c7c5  4. c2c3  4... b8c6  5. g1f3  5... d8b6  6. a2a4 A surprising move early in the game. This isn't listed even in the top five choices of the computer, and it's only the ninth most played move. From what I know, it was never tried by a GM before, so it's a really new concept. To be honest, the move seems quite strange—what's the idea $2 In some lines a4-a5 or Bb5 can be tried, but it seems that after the exchange on d4, the weakened b4-square should give Black good play. Perhaps the greatest merit is forcing the opponent into an unknown position right on move six.