Correct answer:

• b1-e4

White could not avoid losing a piece. They could have played d7 and capture the piece on h4 with their next move, but, because of the GIPF on e7, they would have recycled one piece less than Black. So, they let Black go one more piece ahead, but makes sure to have one extra pieces in reserve.

Black must play g1-g5;x (or i3-g5;x) and finds themself with their pieces scattered all over the board. White plays b1-f4;x and makes sure that Black will be the first to run out of pieces.

That is why c1-c3 or a3-c1 wouldn’t have been good enough, because White needs the piece on b3 to prevent Black from making a capture on the a4-h1 line. If Black plays a4-d4, White answers with a2-d5 and will have no problems to win the game from here: f3 is too isolated and f7 and h4 are separated by white pieces.