Carboxylic acid X forms one ester only with molecular formula C5H10O2,What could X be?

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Dear everyone,I am a student from China. I am facing a problem which I couldn't get throuth. like the below picture.
In my oppinion, the corrrect answer should be A, but the answer from CAIE is c. who can help me? thanks a lot and god bless you

 

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thanks this web and Forums, now I am clear with this question, below is the analysis