Since he didn’t like his dessert, the word is going to have to be a negative taste word.  Subtle doesn’t fit because the sentence already says there is a lot of sugar.  Invigorating is a positive word, so throw it out.  Convoluted means complex and intricate and does not relate to the sentence.   Caustic as a taste word would mean sharp or burning; while it is a negative taste word, a lot of sugar wouldn’t really have that effect.  Unpalatable (think of taste palate) basically means unsavory, and would fit the sentence better.  A is the answer.
 
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