First of all I disagree with Andrew.
d4 is the best move? There is nothing wrong with it and in fact it is nice, but in my opinion 0-0 better. Other nice moves (d4 is in the list)
Depending on what type of play do you prefer you can choose either of them.
0-0 is more aggressive. Idea behind it is that you are already one pawn up and there is no point to save that pawn. So you are developing (hiding your king and preparing to put the rook on e1 attacking the king of the opponent).
If your opponent is eating the pawn
6 ... Nbxd5
Here you are ready for an attack. This type of game if for a strategical opponent who can understand that material is not the most important and opening are for fast developing and getting initiative.
Idea behind d6 is easy. You are trying to maintain this pawn (actually you are forcing to exchange it). The f7 square is under attack and only reasonable way to save it is
6) ... Nbd5 which allows you to capture the pawn
7) dxc7 Qxc7
In the end you are one move from castling (and nothing can stop you) and 1 pawn up. Your opponent is two moves and has one piece more developed than you.
d4, as Andrew said d5 pawn is untouchable, and you are threatening e5 capture with attacking the knight. So if I were black, I would take
6) ... exd4 after which white would castle.
P.S how are you guys adding PNG?