I did a little opening survey of the 190 classical games played so far in this event:
White's opening move:
1. e4 - 85 games, white scores 50.6%
1. d4 - 72 games, white scores 60.4%
1. c4 - 16 games, white scores 59.4%
1. Nf3 - 16 games, white scores 59.4%
1. g3 - 1 game, white scores 50%
Against 1. e4 black plays:
1. ... c5 - 35 games, white scores 48.6%
1. ... e5 - 32 games, white scores 51.6%
1. ... e6 - 9 games, white scores 44.4%
1. ... c6 - 6 games, white scores 50%
1. ... d6 - 2 games, white scores 75%
1. ... d5 - 1 game, white scores 100%
Against 1. d4 black plays:
1. ... Nf6 - 51 games, white scores 60.8%
1. ... d5 - 16 games, white scores 56.3%
1. ... f5 - 2 games, white scores 50%
1. ... e6 - 2 games, white scores 75%
1. ... d6 - 1 game, white scores 100%
Edit: "e4 - most popular by test" just doesn't have quite the right ring to it