Marlowe: This problem is incorrect. It says you cannot play ko. The "winning" move is c18, but black is not supposed to connect at a16, he is supposed to kosumi at a17. Now if white plays C19 like in the solution, black has the extra liberty at a16 to keep it from being self atari. He can surround the white stones and capture and make a living group (w c19, b b17, w a19, b b19) If after a17, white plays b17 instead, then that will go to ko which was prohibited.
Your move has been added.