Chess Opening Theory/1. Nf3/1...d5/2. c4

2. c4 - Réti Opening
This opening is a hypermodern opening, which invites white's minor pieces to control the centre rather than their pawns. Here black has 3 options:

Defending the Pawn
This can be done with moves such as 2... c6, or 2... e6, both of which generally transpose to the Queen's Gambit Declined or Slav Defense. However, white can deviate with 3. g3, leading the positions that resemble the Catalan Opening (or even transposing to the Catalan Opening, normally reached with 1. d4 Nf6 2. c4 e6 3. g3), or even 3. b3!?

Advancing the Pawn
This gives black a spatial advantage in the center and a quite annoying pawn on d4 that controls c3 and e3, squares where the bishop and knight could have originally been developed to. From here, white challenges the pawn with e3, a3 (followed by b4), and b4, while black tries to hold onto the pawn with Nc6 and c5.

The most critical line in this variation is known as the Reversed Blumenfeld Gambit (similar to the Blumenfeld Countergambit played as black), going 3. e3 c5 4. b4 dxe3 5. fxe3 cxb4 6. d4, where black is up a pawn, but it's a weak pawn on b4, and white has central control and development to compensate for that. White is generally considered to get enough compensation for the pawn in this line and gets a ~52% score from it.

White can also choose to avoid this line and go for 3. g3, which leads to positions resembling the Alekhine Defense, where black has a huge spatial advantage in the center after 3... Nc6 4. Bg2 e5 5. O-O e4 6. Ne1, but the pawns are weak and white can attack them with moves like d3. This is best for people who want to follow the "hypermodern" opening ideal, where black follows the classical opening ideas, but white controls the center with pieces and targets the central pawns that black has.

Capturing the Pawn
Capturing the pawn with 2... dxc4 isn't considered the best, but theory and practice has shown that black does just fine. This usually transposes into a Queen's Gambit Accepted line, which is usually fine for both sides. A sample line would be 3. e3 e6 4. Bxc4 c5 5. d4 Nf6, transposing to the Classical Defense of the Queen's Gambit Accepted.

Theory table
1. Nf3 d5