The answer is A).
Option B, it doesn't need a comma after and before "but"
Option C. Doesn't need a comma after "but"
Option D. Has no commas. You should have paused or take a deep breath before finishing up the sentence.
Hence, A) has the correct the correct use of comma and punctuation marks.
A.Is correct. The others should be like this one. B. is wrong because you wouldn't pause after saying but. C. is wrong because you wouldn't say it like this: "But *pause* blah blah blah." D. Is wrong because you pause after you say harpischord. Try saying it out loud and trying these out. You will find that A. makes the most sense.
