9.I9

If 10.H9 11.G10* 12.J8* 13.H8**.

If 10.J9 11.K8* 12.H10** 13.J10*.

White could also have made a threat on the left, for example 9.E7** which would have forced Black to respond on the left. Then White could have followed up with 11.I9. But since it is stipulated that Black resigned after one move by White, the correct answer is 9.I9.

False starts:

A. 9.I8 10.K8.

B. 9.K8 10.I8 11.I9* 12.H10** 13.I7** 14.J11*.