Best response to the Nimzo-Indian Defense: Classical Variation, Noa Variation (E37)

1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. Qc2 d5 5. a3 Bxc3+ 6. Qxc3 Ne4 7. Qc2 · Black to move

What is the best response to the Nimzo-Indian Defense: Classical Variation, Noa Variation (1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. Qc2 d5 5. a3 Bxc3+ 6. Qxc3 Ne4 7. Qc2)? Black to move. See the top replies ranked by opening theory and real-game results, starting with c5.

The main theoretical reply is c5, reaching the Nimzo-Indian Defense: Classical Variation, Modern Variation.