The natural reply, fixing the queenside before White plays a4-a5.
1. d2d4  1... d7d5  2. c2c4  2... e7e6  3. b1c3  3... a7a6  4. c4d5  4... e6d5  5. g1f3  5... g8f6  6. c1g5  6... c8e6  7. e2e3  7... b8d7  8. h2h3  8... f8d6  9. f1d3  9... c7c6  10. g5f4  10... d8c7  11. f4d6  11... c7d6  12. e1g1  12... e8g8 This position has been played many times. Black is very solid, and the pawn on a6, which might have seemed like a waste of time when it was played on the third move, ends up being useful against the typical minority attack with b4-a4-b5. 13. a2a4 [%c_effect a4;square;a4;type;Interesting;persistent;true] I'm not sure if this is an idea straight from Caruana's famous analytical lab or if it was an over-the-board inspiration; the first option seems more likely. This move has been played just once before and is a positionally difficult choice, as Black can fix the pawn structure with a6-a5, which apparently favors him, as the dark-squared bishops have been exchanged.