Explanation:

- Wink refers to the action of closing one eye very briefly, often as a signal or indication.

- Option 1: Sink - Typically means to go down or descend, especially into water.

- Option 2: Close one eye very briefly - This definition accurately captures the act of winking.

- Option 3: Midstream - Refers to being in the middle part of a stream or river, or figuratively, in the middle of a process.

- Option 4: Habit - Stands for a regular practice or routine that is hard to give up.

Based on the meanings, Option 2: Close one eye very briefly is the closest synonym for wink.

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