Answer:
B. Sandomir was reluctant when his sister and aunt asked him to accompany them to the opera, but soon he became enchanted by the lavish sets and passionate singing.
Explanation:
The best version of the given sentence is version B. There are two mistakes in the original sentence:
The highlighted commas should be omitted. When only two things are listed and connected by a coordinating conjunction such as and, the conjunction shouldn't be preceded by a comma.
The only comma that should be used in the sentence is the one before but. But is a coordinating conjunction. When coordinating conjunctions are used to connect two independent clauses, like in this case, they should always be preceded by a comma.