Best way to respond to this opening, It's black to move.
The Ruy Lopez (or Spanish game). 3 ... a6 is by far the most common response, after which the most common continuation is 4.Ba4 Nf6 5.O-O Be7 6.Re1.
what about Nd4?
I'm not fond of it. After 4.Nxd4 exd4, black has weakened his center and has a doubled pawn.
Best way to respond to this opening, It's black to move.