BeginnerExercise193/Solution

Solution  

After , black is alive. If white plays at a, then black can safely tenuki and is still alive.

Not a ko  

If white plays here, then black replies at and lives unconditionally.

Failure  

If black plays here, then after , black must play to live. This is a seki, so black has less territory than in the solution diagram. Alternatively, black could play at a to start a high-risk ko, but that's generally a bad idea.