Black’s best response to White’s c4 opening move; what is the name of this opening?
This is called the English Opening. The objective of the opening is to apply pressure on the center d5 square without committing the queen pawn or the king pawn. The standard response is 1..., e5 (Reversed Sicilian), but there are many other options, 1..., c5 for example (Symmetrical) or 1..., e6, a more manuevering type of game. This opening is known for its transpositional possibilities. For example it often ends up as some form of Queen's Pawn Opening. It is favored by many players because of this non-committal nature.