If we have gone through the previous problems of this series, we will have developed a feeling for this kind of problem and instinctively look for two possibilities: the 1-2 point and the 2-2 point. Here, Black 1 at the 1-2 point is correct. The exchanged marked stones are an indicator.
If White can do anything, she has to prevent the eye at the circled point so she plays 2. After 3, she can only connect at 4.
Black 1 at the 2-2 point is wrong. White 2 gives birth to a bent four in the corner shape and Black dies.
For the sake of repetition, taking a stone with Black 1 is wrong too, for White creates a bulky five shape with 2 and strikes at the vital point with 4. If you have been solving tsumego for a while, 2 and 4 are moves you even don't have to think about anymore.
How about enlarging the eyespace with 1 here ? Well, that's perhaps not too easy to refute and demands some explanation, so I'll make it Gokyo Shumyo, Section 1, Problem 10b.
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