Answer: I love vanilla ice cream, but my sister likes butter pecan.
Explanation: You can use a comma before the coordinating conjunction "but," but not after; the first option is therefore ruled out; however, when using the conjunctive adverb "however" to join two independent clauses, a semicolon should precede it and a comma should follow it (that is why neither the fourth nor the third options are correct).
The second sentence, "I love vanilla ice cream, but my sister likes butter pecan," has the correct punctuation. In a compound sentence, the comma always goes before the coordinating conjunction (In your case, this conjunction was "but.").
