Correct answer:

• a2-f6;x

Through that move White changed Black’s options to recycle pieces drastically. The fact that the piece on c6 is captured, makes that Black lost the possibility to push his piece on d6 onto e6 (x on the e1-e9 diagonal). The other option (i.e. d1-d4;x) is no longer available either. But White offers Black another opportunity to recycle pieces: h1-d4;x. But if Black does so - and he has no other choice -, then White’s next move will be i4-d3;xGe4