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Q: ''Do you have a minute,Ken?''
A: Well, I'm not sure when
B: Sure. What the problem?
C: Good, I hope so
D: Sorry, I haven't got it here
I choose B. But the correction is A. I don't understand. Please explain it me. Thanks!
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Q: ''Do you have a minute,Ken?''
A: Well, I'm not sure when.
B: Sure. What's the problem?
C: Good, I hope so.
D: Sorry, I haven't got it here.
I choose B. But the correction is A. I don't understand. Please explain it me.

In a test like this, you are supposed to choose the most common and likely answer. That is B.

Clive
Comments  
To me, your choice is correct.
Students: We have free audio pronunciation exercises.
 Clive's reply was promoted to an answer.
Sure. What's the problem? (I did not notice the error.)

If the choice B is "What the problem?" and it is not a typo by Tiểu Cầm , then the sentence is also grammatically wrong.