Correct answer:

• a2-d5

The fact that d5 is now occupied by a white piece, causes a threat for the Black GIPF on f4. It makes that Black can’t play at the top of the board, because whatever move he makes there, White will always be able to play either b5 or a3-e7.

But it isn’t any better at the bottom half of the board. If Black would play f1-f3 (or g2, which doesn’t make any difference), White will block the a4-h1 line: a3-g2. Black will have no other choice than f1-f6;xGf6. White won’t even bother about the black pieces on e4 and g2, but play a3-d3 and capture Black’s last GIPF with his next move.