Correct answer:

• a5-d5

This is the only move which ensures White to recycle pieces with his next move. Black won’t play a5-e5 to defend his piece on c4 (White will recycle), neither can he play i1-e5 because that would cost him his last GIPF-piece (white f8-f4;x*f3). So, whatever Black’s move will be, White will always have the possibility to play h6-g6;x, although that will enable Black to counter with f1-f6 and capture 2 of White’s pieces, thus go one more piece ahead.