The solution is to make a clamp at , threatening both a snapback at a as well as connecting underneath at b. Note that after , White at c or d would be self-atari.
Alternatively, may be answered by making an atari at . As captures two stones, both Black and White has two liberties each...
The atari at is wrong. When White plays the hane at , Black is stuck and is unable to make an approach move at a, which is a self-atari.
Note that although is wrong, this failure diagram merits additional discussion. See Kyu Exercise 12 for more.
The hane at is also a mistake, because can make an eye. After , prevents an eye for Black. Just as the previous failure, Black loses by eye versus no eye.