Black's move 5...d6 protects the recently developed knight from being attacked. A pawn move is thus in order, since both knights are developed to good squares, and the Bishop on c8 now has the option, but not the obligation, to develop to a good square once white makes their intentions known.

Of all the responses for white, Bg5 is perhaps the best, and should be expected from a strong opponent. It pins the king knight, prepares for queenside castling, and it is the prelude to the Richter-Rauzer Attack (named after Vsevolod Alfredovich Rauser^[1]).

1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 d6

Wikipedia has related information at Sicilian Defence.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.