Whites most consistent move is 9. d4 freeing the queenside minor pieces and contesting the centre, however, it walks into 9...Bg4 after which the conventional wisdom is that Black has nothing to worry about. For this reason the d4 push is almost always prepared with 9. h3. A few masters have recently attempted to prove that 9. d4 is possible.
White may be fed up with all these preparatory moves and play 9. d3 allowing the safe development of the queenside, but this gives Black complete freedom, claims less space than d4 and limits the scope of the light-squared bishop. The successful chess player must have patience!